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  • 1. Ahn, Kyungjin
    et al.
    Iliev, Ilian T.
    Shapiro, Paul R.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Koda, Jun
    Mao, Yi
    DETECTING THE RISE AND FALL OF THE FIRST STARS BY THEIR IMPACT ON COSMIC REIONIZATION2012In: The Astrophysical Journal Letters, ISSN 2041-8205, Vol. 756, no 1, p. L16-Article in journal (Refereed)
    Abstract [en]

    The intergalactic medium was reionized before redshift z similar to 6, most likely by starlight which escaped from early galaxies. The very first stars formed when hydrogen molecules (H-2) cooled gas inside the smallest galaxies, minihalos (MHs) of mass between 10(5) and 10(8) M-circle dot. Although the very first stars began forming inside these MHs before redshift z similar to 40, their contribution has, to date, been ignored in large-scale simulations of this cosmic reionization. Here we report results from the first reionization simulations to include these first stars and the radiative feedback that limited their formation, in a volume large enough to follow the crucial spatial variations that influenced the process and its observability. We show that, while MH stars stopped far short of fully ionizing the universe, reionization began much earlier with MH sources than without, and was greatly extended, which boosts the intergalactic electron-scattering optical depth and the large-angle polarization fluctuations of the cosmic microwave background significantly. This boost should be readily detectable by Planck, although within current Wilkinson Microwave Anisotropy Probe uncertainties. If reionization ended as late as z(ov) less than or similar to 7, as suggested by other observations, Planck will thereby see the signature of the first stars at high redshift, currently undetectable by other probes.

  • 2. Ahn, Kyungjin
    et al.
    Shapiro, Paul R.
    Iliev, Ilian T.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, Department of Astronomy.
    The Inhomogeneous Background of H2 Dissociating Radiation During Cosmic Reionization2008In: FIRST STARS III: First Stars III Conference, 2008Conference paper (Refereed)
    Abstract [en]

    The first, self-consistent calculations of the cosmological H2 dissociating UV background produced during the epoch of reionization by the sources of reionization are presented. Large-scale radiative transfer simulations of reionization trace the impact of all the ionizing starlight on the IGM from all sources in our simulation volume down to dwarf galaxies of mass ~108 Msolar, identified by very high-resolution N-body simulations, including the self-regulating effect of IGM photoheating on dwarf galaxy formation. The UV continuum emitted below 13.6 eV by each source is then transferred through the same IGM, attenuated by atomic H Lyman series resonance lines, to predict the evolution of the inhomogeneous background in the Lyman-Werner band of H2 between 11 and 13.6 eV.

  • 3. Ahn, Kyungjin
    et al.
    Shapiro, Paul R.
    Iliev, Ilian T.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, Department of Astronomy.
    The Inhomogeneous Background of Hydrogen-Molecule Dissociating Radiation during Cosmic Reionization2008Other (Other academic)
    Abstract [en]

    The first, self-consistent calculations are presented of the cosmological, H2-dissociating UV background produced during the epoch of reionization by the sources of reionization. Large-scale radiative transfer simulations of reionization trace the impact of all the ionizing starlight on the IGM from all the sources in our simulation volume down to dwarf galaxies of mass ~10^8 Msun, identified by very high-resolution N-body simulations, including the self-regulating effect of IGM photoheating on dwarf galaxy formation. The UV continuum emitted below 13.6 eV by each source is then transferred through the same IGM, attenuated by atomic H Lyman series resonance lines, to predict the evolution of the inhomogeneous radiation background in the Lyman-Werner bands of H2 between 11 and 13.6 eV. On average, the intensity of this Lyman-Werner background is found to rise to the threshold level at which dissociation suppresses H2 cooling and star formation inside minihalos, long before reionization is complete. Spatial variations in the Lyman-Werner background are found which result from the clustering of sources associated with large-scale structure formation, such that intensity fluctuations correlate with matter density fluctuations. As a result, the Lyman-Werner background rises to the threshold level for H2 suppression earlier in the vicinity of the reionization sources and their H II regions.

  • 4. Arthur, S. J.
    et al.
    Henney, W. J.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    De Colle, F.
    Vazquez-Semadeni, E.
    Radiation-magnetohydrodynamic simulations of H II regions and their associated PDRs in turbulent molecular clouds2011In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 414, no 2, p. 1747-1768Article in journal (Refereed)
    Abstract [en]

    We present the results of radiation-magnetohydrodynamic simulations of the formation and expansion of H II regions and their surrounding photodissociation regions (PDRs) in turbulent, magnetized, molecular clouds on scales of up to 4 pc. We include the effects of ionizing and non-ionizing ultraviolet radiation and X-rays from population synthesis models of young star clusters. For all our simulations we find that the H II region expansion reduces the disordered component of the magnetic field, imposing a large-scale order on the field around its border, with the field in the neutral gas tending to lie along the ionization front, while the field in the ionized gas tends to be perpendicular to the front. The highest pressure-compressed neutral and molecular gas is driven towards approximate equipartition between thermal, magnetic and turbulent energy densities, whereas lower pressure neutral/molecular gas bifurcates into, on the one hand, quiescent, magnetically dominated regions and, on the other hand, turbulent, demagnetized regions. The ionized gas shows approximate equipartition between thermal and turbulent energy densities, but with magnetic energy densities that are 1-3 orders of magnitude lower. A high velocity dispersion (similar to 8 km s(-1)) is maintained in the ionized gas throughout our simulations, despite the mean expansion velocity being significantly lower. The magnetic field does not significantly brake the large-scale H II region expansion on the length and time-scales accessible to our simulations, but it does tend to suppress the smallest scale fragmentation and radiation-driven implosion of neutral/molecular gas that forms globules and pillars at the edge of the H II region. However, the relative luminosity of ionizing and non-ionizing radiation has a much larger influence than the presence or absence of the magnetic field. When the star cluster radiation field is relatively soft (as in the case of a lower mass cluster, containing an earliest spectral type of B0.5), then fragmentation is less vigorous and a thick, relatively smooth PDR forms.

  • 5. Asad, K. M. B.
    et al.
    Koopmans, L. V. E.
    Jelic, V.
    Pandey, V. N.
    Ghosh, A.
    Abdalla, F. B.
    Bernardi, G.
    Brentjens, M. A.
    de Bruyn, A. G.
    Bus, S.
    Ciardi, B.
    Chapman, E.
    Daiboo, S.
    Fernandez, E. R.
    Harker, G.
    Iliev, I. T.
    Jensen, Hannes
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Martinez-Rubi, O.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mevius, M.
    Offringa, A. R.
    Patil, A. H.
    Schaye, J.
    Thomas, R. M.
    van der Tol, S.
    Vedantham, H. K.
    Yatawatta, S.
    Zaroubi, S.
    Polarization leakage in epoch of reionization windows - I. Low Frequency Array observations of the 3C196 field2015In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 451, no 4, p. 3709-3727Article in journal (Refereed)
    Abstract [en]

    Detection of the 21-cm signal coming from the epoch of reionization (EoR) is challenging especially because, even after removing the foregrounds, the residual Stokes <italic toggle=yes>I maps contain leakage from polarized emission that can mimic the signal. Here, we discuss the instrumental polarization of Low Frequency Array (LOFAR) and present realistic simulations of the leakages between Stokes parameters. From the LOFAR observations of polarized emission in the 3C196 field, we have quantified the level of polarization leakage caused by the nominal model beam of LOFAR, and compared it with the EoR signal using power spectrum analysis. We found that at 134-166 MHz, within the central 4A degrees of the field the (<italic toggle=yes>Q, <italic toggle=yes>U) -> <italic toggle=yes>I leakage power is lower than the EoR signal at <italic toggle=yes>k < 0.3 Mpc(-1). The leakage was found to be localized around a Faraday depth of 0, and the rms of the leakage as a fraction of the rms of the polarized emission was shown to vary between 0.2 and 0.3 per cent, both of which could be utilized in the removal of leakage. Moreover, we could define an 'EoR window' in terms of the polarization leakage in the cylindrical power spectrum above the point spread function (PSF)-induced wedge and below <italic toggle=yes>k(ayen) similar to 0.5 Mpc(-1), and the window extended up to <italic toggle=yes>k(ayen) similar to 1 Mpc(-1) at all <italic toggle=yes>k(aSyen) when 70 per cent of the leakage had been removed. These LOFAR results show that even a modest polarimetric calibration over a field of view of a parts per thousand(2) 4A degrees in the future arrays like Square Kilometre Array will ensure that the polarization leakage remains well below the expected EoR signal at the scales of 0.02-1 Mpc(-1).

  • 6. Ciardi, B.
    et al.
    Inoue, S.
    Abdalla, F. B.
    Asad, K.
    Bernardi, G.
    Bolton, J. S.
    Brentjens, M.
    de Bruyn, A. G.
    Chapman, E.
    Daiboo, S.
    Fernandez, E. R.
    Ghosh, A.
    Graziani, L.
    Harker, G. J. A.
    Iliev, I. T.
    Jelic, V.
    Jensen, Hannes
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kazemi, S.
    Koopmans, L. V. E.
    Martinez, O.
    Maselli, A.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Offringa, A. R.
    Pandey, V. N.
    Schaye, J.
    Thomas, R.
    Vedantham, H.
    Yatawatta, S.
    Zaroubi, S.
    Simulating the 21 cm forest detectable with LOFAR and SKA in the spectra of high-z GRBs2015In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 453, no 1, p. 101-105Article in journal (Refereed)
    Abstract [en]

    We investigate the feasibility of detecting 21 cm absorption features in the afterglow spectra of high redshift long Gamma Ray Bursts (GRBs). This is done employing simulations of cosmic reionization, together with estimates of the GRB radio afterglow flux and the instrumental characteristics of the LOw Frequency ARray (LOFAR). We find that absorption features could be marginally (with a S/N larger than a few) detected by LOFAR at z greater than or similar to 7 if the GRB is a highly energetic event originating from Pop III stars, while the detection would be easier if the noise were reduced by one order of magnitude, i.e. similar to what is expected for the first phase of the Square Kilometre Array (SKA1-low). On the other hand, more standard GRBs are too dim to be detected even with ten times the sensitivity of SKA1-low, and only in the most optimistic case can a S/N larger than a few be reached at z greater than or similar to 9.

  • 7. Ciardi, B.
    et al.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Yatawatta, S.
    Prospects for detecting the 21 cm forest from the diffuse intergalactic medium with LOFAR2013In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 428, no 2, p. 1755-1765Article in journal (Refereed)
    Abstract [en]

    We discuss the feasibility of the detection of the 21 cm forest in the diffuse intergalactic medium (IGM) with the radio telescope LOFAR. The optical depth to the 21 cm line has been derived using simulations of reionization which include detailed radiative transfer of ionizing photons. We find that the spectra from reionization models with similar total comoving hydrogen ionizing emissivity but different frequency distribution look remarkably similar. Thus, unless the reionization histories are very different from each other (e.g. a predominance of UV versus X-ray heating) we do not expect to distinguish them by means of observations of the 21 cm forest. Because the presence of a strong X-ray background would make the detection of the 21 cm line absorption impossible, the lack of absorption could be used as a probe of the presence/intensity of the X-ray background and the thermal history of the Universe. Along a random line of sight LOFAR could detect a global suppression of the spectrum from z greater than or similar to 12, when the IGM is still mostly neutral and cold, in contrast with the more well-defined, albeit broad, absorption features visible at lower redshift. Sharp, strong absorption features associated with rare, high-density pockets of gas could also be detected at z similar to 7 along preferential lines of sight.

  • 8.
    Datta, Kanan K.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Friedrich, Martina M.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Iliev, Ilian T.
    Shapiro, Paul R.
    Prospects of observing a quasar H ii region during the epoch of reionization with the redshifted 21-cm signal2012In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 424, no 1, p. 762-778Article in journal (Refereed)
    Abstract [en]

    We present a study of the impact of a bright quasar on the redshifted 21-cm signal during the epoch of reionization (EoR). Using three different cosmological radiative transfer simulations, we investigate if quasars are capable of substantially changing the size and morphology of the H ii regions they are born in. We choose stellar and quasar luminosities in a way that is favourable to seeing such an effect. We find that even the most luminous of our quasar models is not able to increase the size of its native H ii region substantially beyond those of large H ii regions produced by clustered stellar sources alone. However, the quasar H ii region is found to be more spherical. We next investigate the prospects of detecting such H ii regions in the redshifted 21-cm data from the Low Frequency Array (LOFAR) by means of a matched filter technique. We find that H ii regions with radii similar to 25 comoving Mpc or larger should have a sufficiently high detection probability for 1200 h of integration time. Although the matched filter can in principle distinguish between more and less spherical regions, we find that when including realistic system noise this distinction can no longer be made. The strong foregrounds are found not to pose a problem for the matched filter technique. We also demonstrate that when the quasar position is known, the redshifted 21-cm data can still be used to set upper limits on the ionizing photon rate of the quasar. If both the quasar position and its luminosity are known, the redshifted 21-cm data can set new constraints on quasar lifetimes.

  • 9.
    Datta, Kanan K.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). National Centre for Radio Astrophysics, India.
    Jensen, Hannes
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Majumdar, Suman
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Iliev, Ilian T.
    Mao, Yi
    Shapiro, Paul R.
    Ahn, Kyungjin
    Light cone effect on the reionization 21-cm signal - II. Evolution, anisotropies and observational implications2014In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 442, no 2, p. 1491-1506Article in journal (Refereed)
    Abstract [en]

    Measurements of the H i 21-cm power spectra from the reionization epoch will be influenced by the evolution of the signal along the line-of-sight direction of any observed volume. We use numerical as well as seminumerical simulations of reionization in a cubic volume of 607 Mpc across to study this so-called light-cone effect on the H i 21-cm power spectrum. We find that the light-cone effect has the largest impact at two different stages of reionization: one when reionization is similar to 20 per cent and other when it is similar to 80 per cent completed. We find a factor of similar to 4 amplification of the power spectrum at the largest scale available in our simulations. We do not find any significant anisotropy in the 21-cm power spectrum due to the light-cone effect. We argue that for the power spectrum to become anisotropic, the light-cone effect would have to make the ionized bubbles significantly elongated or compressed along the line of sight, which would require extreme reionization scenarios. We also calculate the two-point correlation functions parallel and perpendicular to the line of sight and find them to differ. Finally, we calculate an optimum frequency bandwidth below which the light-cone effect can be neglected when extracting power spectra from observations. We find that if one is willing to accept a 10 per cent error due to the light-cone effect, the optimum frequency bandwidth for k = 0.056 Mpc(-1) is similar to 7.5 MHz. For k = 0.15 and 0.41 Mpc(-1), the optimum bandwidth is similar to 11 and similar to 16 MHz, respectively.

  • 10. de Val-Borro, Miguel
    et al.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy.
    Peplinski, Adam
    Stockholm University, Faculty of Science, Department of Astronomy.
    A Comparison of Numerical Simulations of Disk-Planet Interactions2007In: American Astronomical Society, DPS meeting #39, #42.05, 2007Conference paper (Other academic)
  • 11. Dixon, Keri L.
    et al.
    Iliev, Ilian T.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy.
    Ahn, Kyungjin
    Shapiro, Paul R.
    The large-scale observational signatures of low-mass galaxies during reionization2016In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 456, no 3, p. 3011-3029Article in journal (Refereed)
    Abstract [en]

    Observations of the epoch of reionization give us clues about the nature and evolution of the sources of ionizing photons, or early stars and galaxies. We present a new suite of structure formation and radiative transfer (RT) simulations from the PRACE4LOFAR project designed to investigate whether the mechanism of radiative feedback, or the suppression of star formation in ionized regions from UV radiation, can be inferred from these observations. Our source halo mass extends down to 10(8) M-circle dot, with sources in the mass range 10(8)-10(9) M-circle dot expected to be particularly susceptible to feedback from ionizing radiation, and we vary the aggressiveness and nature of this suppression. Not only do we have four distinct source models, we also include two box sizes (67 and 349 Mpc), each with two grid resolutions. This suite of simulations allows us to investigate the robustness of our results. All of our simulations are broadly consistent with the observed electron-scattering optical depth of the cosmic microwave background and the neutral fraction and photoionization rate of hydrogen at z similar to 6. In particular, we investigate the redshifted 21-cm emission in anticipation of upcoming radio interferometer observations. We find that the overall shape of the 21-cm signal and various statistics are robust to the exact nature of source suppression, the box size, and the resolution. There are some promising model discriminators in the non-Gaussianity and small-scale power spectrum of the 21-cm signal.

  • 12. Dore, Olivier
    et al.
    Holder, Gil
    Alvarez, Marcelo
    Iliev, Ilian T.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy.
    Signature of patchy reionization in the polarization anisotropy of the CMB2007In: Physical Review D, Vol. 76, no 4, p. 043002-Article in journal (Refereed)
  • 13. Fernandez, Elizabeth R.
    et al.
    Zaroubi, Saleem
    Iliev, Ilian T.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Jelic, Vibor
    Stars and reionization: the cross-correlation of the 21 cm line and the near-infrared background2014In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 440, no 1, p. 298-306Article in journal (Refereed)
    Abstract [en]

    With improving telescopes, it may now be possible to observe the Epoch of Reionization in multiple ways. We examine two of these observables - the excess light in the near-infrared background that may be due to high-redshift stars and ionized Hii bubbles, and the 21 cm emission from neutral hydrogen. Because these two forms of emission should result from different, mutually exclusive regions, an anticorrelation should exist between them. We discuss the strengths of using cross-correlations between these observations to learn more about high-redshift star formation and reionization history. In particular, we create simulated maps of emission from both the near-infrared background and 21 cm emission. We find that these observations are anticorrelated, with the strongest anticorrelation originating from times when the universe is half ionized. This result is robust and does not depend on the properties of the stars themselves. Rather, it depends on the ionization history. Cross-correlations can provide redshift information, which the near-infrared background cannot provide alone. In addition, cross-correlations can help separate foreground emission from the true high-redshift component, making it possible to say with greater certainty that we are indeed witnessing the Epoch of Reionization.

  • 14.
    Friedrich, Martina M.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Alvarez, Marcelo A.
    Shapiro, Paul R.
    Iliev, Ilian T.
    Topology and sizes of H II regions during cosmic reionization2011In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 413, no 2, p. 1353-1372Article in journal (Refereed)
    Abstract [en]

    We use the results of large-scale simulations of reionization to explore methods for characterizing the topology and sizes of H ii regions during reionization. We use four independent methods for characterizing the sizes of ionized regions. Three of them give us a full size distribution: the friends-of-friends (FoF) method, the spherical average (SPA) method and the power spectrum (PS) of the ionized fraction. The latter three methods are complementary: while the FoF method captures the size distribution of the small-scale H ii regions, which contribute only a small amount to the total ionization fraction, the SPA method provides a smoothed measure for the average size of the H ii regions constituting the main contribution to the ionized fraction, and the PS does the same while retaining more details on the size distribution. Our fourth method for characterizing the sizes of the H ii regions is the average size which results if we divide the total volume of the H ii regions by their total surface area (i.e. 3V/A), computed in terms of the ratio of the corresponding Minkowski functionals of the ionized fraction field. To characterize the topology of the ionized regions, we calculate the evolution of the Euler characteristic. We find that the evolution of the topology during the first half of reionization is consistent with inside-out reionization of a Gaussian density field. We use these techniques to investigate the dependence of the size and topology on some basic source properties, such as the halo mass-to-light ratio, susceptibility of haloes to negative feedback from reionization and the minimum halo mass for sources to form. We find that the suppression of ionizing sources within ionized regions slows the growth of H ii regions and also changes their size distribution. Additionally, the topology of simulations including suppression is more complex, as indicated by the evolution of the Euler characteristic of the ionized regions. We find the density and ionized fraction to be correlated on large scales, in agreement with the inside-out picture of reionization.

  • 15.
    Friedrich, Martina M.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy.
    Datta, Kanan K.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Iliev, Ilian T.
    Prospects of observing a quasar HII region during the Epoch ofReionization with redshifted 21cmIn: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966Article in journal (Refereed)
    Abstract [en]

    We present a study of the impact of a bright quasar on the redshifted 21cm signal during theEpoch of Reionization (EoR). Using three different cosmological radiative transfer simula-tions, we investigate if quasars are capable of substantially changing the size and morphologyof the H II regions they are born in. We choose stellar and quasar luminosities in a way that isfavourable to seeing such an effect. We find that even the most luminous of our quasar modelsis not able to increase the size of its native H II region substantially beyond those of largeH II regions produced by clustered stellar sources alone. However, the quasar H II region isfound to be more spherical. We next investigate the prospects of detecting such H II regionsin the redshifted 21cm data from the Low Frequency Array (LOFAR) by means of a matchedfilter technique. We find that H II regions with radii ∼ 25 comoving Mpc or larger shouldhave a sufficiently high detection probability for 1200 hours of integration time. Although thematched filter can in principle distinguish between more and less spherical regions, we findthat when including realistic system noise this distinction can no longer be made. The strongforegrounds are found not to pose a problem for the matched filter technique. We also demon-strate that when the quasar position is known, the redshifted 21cm data can still be used toset upper limits on the ionizing photon rate of the quasar. If both the quasar position and itsluminosity are known, the redshifted 21 cm data can set new constrains on quasar lifetimes.

  • 16.
    Friedrich, Martina M.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Iliev, Ilian T.
    Shapiro, Paul R.
    Radiative Transfer of energetic photons: X-rays and helium ionization in C2-Ray2012In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 421, no 3, p. 2232-2250Article in journal (Refereed)
    Abstract [en]

    We present an extension to the short-characteristic ray-tracing and non-equilibrium photoionization code C2-RAY. The new version includes the effects of helium and improved multifrequency heating. The motivation for this work is to be able to deal with harder ionizing spectra, such as from quasar-like sources during cosmic reionization. We review the basic algorithmic ingredients of C2-RAY before describing the changes implemented, which include a treatment of the full on-the-spot (OTS) approximation, secondary ionization, and multifrequency photoionization and heating. We performed a series of tests against equilibrium solutions from CLOUDY as well as comparisons to the hydrogen-only solutions by C2-RAY in the extensive cosmological radiative transfer code comparison project. We show that the full, coupled OTS approximation is more accurate than the simplified, uncoupled one. We find that also with helium and a multifrequency setup, long time-steps (up to ~10 per cent of the recombination time) still give accurate results for the ionization fractions. On the other hand, accurate results for the temperature set strong constraints on the time-step. The details of these constraints depend, however, on the optical depth of the cells. We use the new version of the code to confirm that the assumption made in many reionization simulations, namely that helium is singly ionized everywhere where hydrogen is, is indeed valid when the sources have stellar-like spectra.

  • 17.
    Giri, Sambit K.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Dixon, Keri L.
    Iliev, Ilian T.
    Bubble size statistics during reionization from 21-cm tomography2018In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 473, no 3, p. 2949-2964Article in journal (Refereed)
    Abstract [en]

    The upcoming SKA1-Low radio interferometer will be sensitive enough to produce tomographic imaging data of the redshifted 21-cm signal from the Epoch of Reionization. Due to the non-Gaussian distribution of the signal, a power spectrum analysis alone will not provide a complete description of its properties. Here, we consider an additional metric which could be derived from tomographic imaging data, namely the bubble size distribution of ionized regions. We study three methods that have previously been used to characterize bubble size distributions in simulation data for the hydrogen ionization fraction-the spherical-average (SPA), mean-free-path (MFP) and friends-of-friends (FOF) methods -and apply them to simulated 21-cm data cubes. Our simulated data cubes have the (sensitivity-dictated) resolution expected for the SKA1-Low reionization experiment and we study the impact of both the light-cone (LC) and redshift space distortion (RSD) effects. To identify ionized regions in the 21-cm data we introduce a new, self-adjusting thresholding approach based on the K-Means algorithm. We find that the fraction of ionized cells identified in this way consistently falls below the mean volume-averaged ionized fraction. From a comparison of the three bubble size methods, we conclude that all three methods are useful, but that the MFP method performs best in terms of tracking the progress of reionization and separating different reionization scenarios. The LC effect is found to affect data spanning more than about 10 MHz in frequency (Delta z similar to 0.5). We find that RSDs only marginally affect the bubble size distributions.

  • 18. Griffen, B. F.
    et al.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    The inhomogeneous reionization of the local intergalactic medium by metal-poor globular clusters2013In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 431, no 4, p. 3087-3102Article in journal (Refereed)
    Abstract [en]

    We present detailed radiative transfer simulations of the reionization of the Milky Way by metal-poor globular clusters. We identify potential metal-poor globular cluster candidates within the Aquarius simulation using dark matter halo velocity dispersions. We calculate the local ionization fields via a photon-conserving, three dimensional non-equilibrium chemistry code. The key feature of the model is that globular cluster formation is suppressed if the local gas is ionized. We assume that at these early times, the ionization field is dominated by the flux from metal-poor globular clusters. Our spatial treatment of the ionization field leads to drastically different numbers and spatial distributions when compared to models where globular cluster formation is simply truncated at early redshifts (z similar to 13). The spatial distributions are more extended and more globular clusters are produced. We find that additional sources of ionization are required at later epochs (z similar to 10) to ionize the remaining gas and recover radial distributions statistically consistent with that of the Milky Way metal-poor globular clusters. We investigate a range of plausible ionization efficiencies to determine the effect photon-rich and photon-poor models have on present-day globular cluster properties. If globular clusters do indeed form within high-redshift dark matter haloes, they produce enough photons to ionize 98 and 90 per cent local (i.e. 2(3) h(-3) Mpc(3) centred on the host galaxy) volume and mass by redshift 10, respectively. In our photon-poorest model, this contribution drops to 60 and 50 per cent. Our model therefore implies that globular clusters are important contributors to the reionization process on local scales at high-redshift until more photon-rich sources dominate the photon budget at later times. The surviving clusters in all models have a narrow average age range (mean = 13.34 Gyr, sigma = 0.04 Gyr) consistent with current age estimates of the Milky Way metal-poor globular clusters. We also test a simple dynamical destruction model and estimate that similar to 60 per cent of all metal-poor globular clusters formed at high redshift have since been destroyed via tidal interactions with the host galaxy.

  • 19. Harker, Geraint
    et al.
    Zaroubi, Saleem
    Bernardi, Gianni
    Brentjens, Michiel A.
    de Bruyn, A. G.
    Ciardi, Benedetta
    Jelic, Vibor
    Koopmans, Leon V. E.
    Labropoulos, Panagiotis
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Offringa, Andre
    Pandey, V. N.
    Pawlik, Andreas H.
    Schaye, Joop
    Thomas, Rajat M.
    Yatawatta, Sarod
    Power spectrum extraction for redshifted 21-cm Epoch of Reionization experiments: the LOFAR case2010In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 405, no 4, p. 2492-2504Article in journal (Refereed)
    Abstract [en]

    One of the aims of the Low Frequency Array (LOFAR) Epoch of Reionization (EoR) project is to measure the power spectrum of variations in the intensity of redshifted 21-cm radiation from the EoR. The sensitivity with which this power spectrum can be estimated depends on the level of thermal noise and sample variance, and also on the systematic errors arising from the extraction process, in particular from the subtraction of foreground contamination. We model the extraction process using realistic simulations of the cosmological signal, the foregrounds and noise, and so estimate the sensitivity of the LOFAR EoR experiment to the redshifted 21-cm power spectrum. Detection of emission from the EoR should be possible within 360 h of observation with a single station beam. Integrating for longer, and synthesizing multiple station beams within the primary (tile) beam, then enables us to extract progressively more accurate estimates of the power at a greater range of scales and redshifts. We discuss different observational strategies which compromise between depth of observation, sky coverage and frequency coverage. A plan in which lower frequencies receive a larger fraction of the time appears to be promising. We also study the nature of the bias which foreground fitting errors induce on the inferred power spectrum and discuss how to reduce and correct for this bias. The angular and line-of-sight power spectra have different merits in this respect, and we suggest considering them separately in the analysis of LOFAR data.

  • 20. Hong, Sungwook E.
    et al.
    Ahn, Kyungjin
    Park, Changbom
    Kim, Juhan
    Iliev, Ilian T.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    2D GENUS TOPOLOGY OF 21-CM DIFFERENTIAL BRIGHTNESS TEMPERATURE DURING COSMIC REIONIZATION2014In: Journal of the Korean Astronomical Society, ISSN 1225-4614, Vol. 47, no 2, p. 49-67Article in journal (Refereed)
    Abstract [en]

    A novel method to characterize the topology of the early-universe intergalactic medium during the epoch of cosmic reionization is presented. The 21-cm radiation background from high redshift is analyzed through calculation of the 2-dimensional (2D) genus. The radiative transfer of hydrogenionizing photons and ionization-rate equations are calculated in a suite of numerical simulations under various input parameters. The 2D genus is calculated from the mock 21-cm images of high-redshift universe. We construct the 2D genus curve by varying the threshold differential brightness temperature, and compare this to the 2D genus curve of the underlying density field. We find that (1) the 2D genus curve reflects the evolutionary track of cosmic reionization and (2) the 2D genus curve can discriminate between certain reionization scenarios and thus indirectly probe the properties of radiation-sources. Choosing the right beam shape of a radio antenna is found crucial for this analysis. Square Kilometre Array (SKA) is found to be a suitable apparatus for this analysis in terms of sensitivity, even though some deterioration of the data for this purpose is unavoidable under the planned size of the antenna core.

  • 21. Iliev, Ilian T.
    et al.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy.
    Character and detectability of the dark ages and the epoch of reionization: the view from the simulations2007Report (Other academic)
  • 22. Iliev, Ilian T.
    et al.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy.
    Current models of the observable consequences of cosmic reionization and their detectability2007Report (Other academic)
  • 23. Iliev, Ilian T.
    et al.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy.
    Effect of the intergalactic environment on the observability of Ly-alpha emitters during reionization2007Report (Other academic)
  • 24. Iliev, Ilian T.
    et al.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, Department of Astronomy.
    Simulating Reionization: Character and Observability2007Report (Other academic)
  • 25. Iliev, Ilian T.
    et al.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Ahn, Kyungjin
    Shapiro, Paul R.
    Mao, Yi
    Pen, Ue-Li
    Simulating cosmic reionization: how large a volume is large enough?2014In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 439, no 1, p. 725-743Article in journal (Refereed)
    Abstract [en]

    We present the largest-volume (425 Mpc h(-1) = 607 Mpc on a side) full radiative transfer simulation of cosmic reionization to date. We show that there is significant additional power in density fluctuations at very large scales. We systematically investigate the effects this additional power has on the progress, duration and features of reionization and on selected reionization observables. We find that comoving volume of similar to 100 Mpc h(-1) per side is sufficient for deriving a convergent mean reionization history, but that the reionization patchiness is significantly underestimated. We use jackknife splitting to quantify the convergence of reionization properties with simulation volume. We find that sub-volumes of similar to 100 Mpc h(-1) per side or larger yield convergent reionization histories, except for the earliest times, but smaller volumes of similar to 50 Mpc h(-1) or less are not well converged at any redshift. Reionization history milestones show significant scatter between the sub-volumes, as high as Delta z similar to 1 for similar to 50 Mpc h(-1) volumes. If we only consider mean-density sub-regions the scatter decreases, but remains at Delta z similar to 0.1-0.2 for the different size sub-volumes. Consequently, many potential reionization observables like 21-cm rms, 21-cm PDF skewness and kurtosis all showgood convergence for volumes of similar to 200 Mpc h(-1), but retain considerable scatter for smaller volumes. In contrast, the three-dimensional 21-cm power spectra at large scales (k < 0.25 h Mpc(-1)) do not fully converge for any sub-volume size. These additional large-scale fluctuations significantly enhance the 21-cm fluctuations, which should improve the prospects of detection considerably, given the lower foregrounds and greater interferometer sensitivity at higher frequencies.

  • 26. Iliev, Ilian T.
    et al.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy.
    Pen, Ue-Li
    Bond, Richard J.
    Shapiro, Paul R.
    Current models of the observable consequences of cosmic reionization and their detectability2008In: Monthly Notices of the Royal Astronomical Society, ISSN 00358711, Vol. 384, no 3, p. 863-874p. 863-874Article in journal (Refereed)
    Abstract [en]

    A number of large current experiments aim to detect the signatures of the cosmic reionization at redshifts z > 6. Their success depends crucially on understanding the character of the reionization process and its observable consequences and designing the best strategies to use. We use large-scale simulations of cosmic reionization to evaluate the reionization signatures at redshifted 21-cm and small-scale cosmic microwave background (CMB) anisotropies in the best current model for the background universe, with fundamental cosmological parameters given by Wilkinson Microwave Anisotropy Probe three-year results. We find that the optimal frequency range for observing the ‘global step’ of the 21-cm emission is 120–150 MHz, while statistical studies should aim at 140–160 MHz, observable by GMRT. Some strongly non-Gaussian brightness features should be detectable at frequencies up to ∼190 MHz. In terms of sensitivity-signal trade-off relatively low resolutions, corresponding to beams of at least a few arcminutes, are preferable. The CMB anisotropy signal from the kinetic Sunyaev–Zel'dovich effect from reionized patches peaks at tens of μK at arcminute scales and has an rms of ∼1 μK, and should be observable by the Atacama Cosmology Telescope and the South Pole Telescope. We discuss the various observational issues and the uncertainties involved, mostly related to the poorly known reionization parameters and, to a lesser extend, to the uncertainties in the background cosmology.

  • 27. Iliev, Ilian T.
    et al.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy.
    Shapiro, Paul R.
    Pen, Ue-Li
    Self-Regulated Reionization2007In: Monthly Notices of the Royal Astronomical Society, Vol. 376, no 2, p. 534-548Article in journal (Refereed)
    Abstract [en]

    Recently, we have presented the first, truly large-scale radiative transfer simulations of inhomogeneous cosmic reionization which resolve all the possible halo sources down to the dwarf galaxy mass range, M > 109 Mo, in a comoving volume (100 h-1 Mpc)3. This is large enough to sample the global mean history, geometry and statistical properties of reionization fairly and accurately for the first time. Here we present new simulations which extend the source halo mass range downward to 108 Mo, to capture the full range of halo masses thought to be primarily responsible for reionization by their star formation following atomic hydrogen radiative cooling and gravitational collapse. Haloes below about 109 Mo, however, are subject to Jeans-mass filtering in the ionized regions, which suppresses their baryonic content and their ability to release ionizing radiation. By including these smaller mass haloes but accounting for their suppression, too, we find that reionization is ‘self-regulating’, as follows. As the mean ionized fraction rises, so does the fraction of the volume within which suppression occurs. Hence, the degree of suppression is related to the mean ionized fraction. Since low-mass haloes with high efficiency (i.e. high emissivity) achieve a given mean ionized fraction earlier than do those with low efficiency, Jeans-mass filtering compensates for the difference in the emissivity of the suppressible haloes in these two cases. As a result, in the presence of lower mass source haloes, reionization begins earlier, but the later stages of reionization and the time of overlap are dictated by the efficiency of the higher mass haloes, independent of the efficiency of the suppressible, lower mass haloes. Hence, while the lower mass haloes do not alter the overlap redshift, zov, they serve to boost the electron-scattering optical depth of the universe, τes. This may explain why observations of quasar absorption spectra at high redshift find that reionization ended late (zov < 7), while cosmic microwave background polarization measurements report a large enough τes that reionization must have begun much earlier (z > 11). We present results for the Lambda cold dark matter universe with cosmological parameters from both 1- and 3-yr data releases of Wilkinson Microwave Anisotropy Probe. Reionization histories consistent with current constraints on zov and τes are shown to be achievable with standard stellar sources in haloes above 108 Mo. Neither minihaloes nor exotic sources are required, and the phenomenon of ‘double reionization’ previously suggested does not occur.

  • 28. Iliev, Ilian T.
    et al.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Shapiro, Paul R.
    Pen, Ue-Li
    Mao, Yi
    Koda, Jun
    Ahn, Kyungjin
    Can 21-cm observations discriminate between high-mass and low-mass galaxies as reionization sources?2012In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 423, no 3, p. 2222-2253Article in journal (Refereed)
    Abstract [en]

    The prospect of detecting the first galaxies by observing their impact on the intergalactic medium (IGM) as they reionized it during the first billion years leads us to ask whether such indirect observations are capable of diagnosing which types of galaxies were most responsible for reionization. We attempt to answer this with new large-scale radiative transfer simulations of reionization including the entire mass range of atomically cooling haloes (M > 108 M?). We divide these haloes into two groups, high-mass, atomically cooling haloes, or HMACHs (M > 109 M?), and low-mass, atomically cooling haloes, or LMACHs (108 < M < 109 M?), the latter being susceptible to negative feedback due to Jeans mass filtering in ionized regions, which leads to a process we refer to as self-regulation. We focus here on predictions of the redshifted 21-cm emission, to see if upcoming observations are capable of distinguishing a universe ionized primarily by HMACHs from one in which both HMACHs and LMACHs are responsible, and to see how these results depend upon the uncertain source efficiencies. We find that 21-cm fluctuation power spectra observed by the first-generation Epoch of Reionization 21-cm radio interferometer arrays should be able to distinguish the case of reionization by HMACHs alone from that by both HMACHs and LMACHs, together. Some reionization scenarios, e.g. one with abundant low-efficiency sources versus one with self-regulation, yield very similar power spectra and rms evolution and thus can only be discriminated by their different mean reionization history and 21-cm probability distribution function (PDF) distributions. We also find that the skewness of the 21-cm PDF distribution smoothed with Low Frequency Array (LOFAR)-like resolution shows a clear feature correlated with the rise of the rms due to patchiness. This is independent of the reionization scenario and thus provides a new approach for detecting the rise of large-scale patchiness. The peak epoch of the 21-cm rms fluctuations depends significantly on the beam and bandwidth smoothing size as well as on the reionization scenario and can occur for ionized fractions as low as 30 per cent and as high as 70 per cent. Measurements of the mean photoionization rates are sensitive to the average density of the regions being studied and therefore could be strongly skewed in certain cases. Finally, the simulation volume employed has very modest effects on the results during the early and intermediate stages of reionization, but late-time signatures could be significantly affected.

  • 29. Iliev, Ilian T.
    et al.
    Moore, Ben
    Gottloeber, Stefan
    Yepes, Gustavo
    Hoffman, Yehuda
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy.
    Reionization of the Local Group of galaxies2011In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 413, no 3, p. 2093-2102Article in journal (Refereed)
    Abstract [en]

    We present the first detailed structure formation and radiative transfer simulations of the reionization history of our cosmic neighbourhood. To this end, we follow the formation of the Local Group of galaxies and nearby clusters by means of constrained simulations, which use the available observational constraints to construct a representation of those structures which reproduces their actual positions and properties at the present time. We find that the reionization history of the Local Group is strongly dependent on the assumed photon production efficiencies of the ionizing sources, which are still poorly constrained. If sources are relatively efficient, i.e. the process is 'photon-rich', the Local Group is primarily ionized externally by the nearby clusters. Alternatively, if the sources are inefficient, i.e. reionization is 'photon-poor' the Local Group evolves largely isolated and reionizes itself. The mode of reionization, external versus internal, has important implications for the evolution of our neighbourhood, in terms of e.g. its satellite galaxy populations and primordial stellar populations. This therefore provides an important avenue for understanding the young universe by detailed studies of our nearby structures.

  • 30. Iliev, Ilian T.
    et al.
    Pen, Ue-Li
    Bond, J. Richard
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy.
    Shapiro, Paul R.
    kSZ from patchy reionization: The view from the simulations2006In: New Astronomy Reviews, Vol. 50, no 11-12, p. 909-917Article in journal (Refereed)
    Abstract [en]

    We present the first calculation of the kinetic Sunyaev Zel’dovich (kSZ) effect due to the inhomogeneus reionization of the universe based on detailed large-scale radiative transfer simulations of reionization. The resulting sky power spectra peak at ℓ = 2000 8000 with maximum values of [ℓ(ℓ + 1)Cℓ/(2π)]max ˜ 4 7 × 10^‑13. The scale roughly corresponds to the typical ionized bubble sizes observed in our simulations, of ˜5 20 Mpc. The kSZ anisotropy signal from reionization dominates the primary CMB signal above ℓ = 3000. At large-scales the patchy kSZ signal depends only on the source efficiencies. It is higher when sources are more efficient at producing ionizing photons, since such sources produce larger ionized regions, on average, than less efficient sources. The introduction of sub-grid gas clumping in the radiative transfer simulations produce significantly more power at small-scales, but has little effect at large-scales. The patchy reionization kSZ signal is dominated by the post-reionization signal from fully-ionized gas, but the two contributions are of similar order at scales ℓ ˜ 3000 ‑ 10^4, indicating that the kSZ anisotropies from reionization are an important component of the total kSZ signal at these scales.

  • 31. Iliev, Ilian T.
    et al.
    Pen, Ue-Li
    McDonald, Patrick
    Shapiro, Paul R.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy.
    Alvarez, Marcelo A.
    Reionization: characteristic scales, topology and observability2009In: Astrophysics and Space Science, ISSN 0004-640X, E-ISSN 1572-946X, Vol. 320, no 1-3, p. 39-43Article in journal (Refereed)
    Abstract [en]

    Recently the numerical simulations of the process of reionization of the universe at z>6 have made a qualitative leap forward, reaching sufficient sizes and dynamic range to determine the characteristic scales of this process. This allowed making the first realistic predictions for a variety of observational signatures. We discuss recent results from large-scale radiative transfer and structure formation simulations on the observability of high-redshift Ly- α sources. We also briefly discuss the dependence of the characteristic scales and topology of the ionized and neutral patches on the reionization parameters.

  • 32. Iliev, Ilian T.
    et al.
    Shapiro, Paul R.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, Department of Astronomy.
    Simulating Cosmic Reionization2008Other (Other academic)
    Abstract [en]

    The Cosmic Dark Ages and the Epoch of Reionization constitute a crucial missing link in our understanding of the evolution of the intergalactic medium and the formation and evolution of galaxies. Due to the complex nature of this global process it is best studied through large-scale numerical simulations. This presents considerable computational challenges. The dominant contributors of ionizing radiation were dwarf galaxies. These tiny galaxies must be resolved in very large cosmological volumes in order to derive their clustering properties and the corresponding observational signatures correctly, which makes this one of the most challenging problems of numerical cosmology. We have recently performed the largest and most detailed simulations of the formation of early cosmological large-scale structures and their radiative feedback leading to cosmic reionization. This was achieved by running extremely large (up to 29 billion-particle) N-body simulations of the formation of the Cosmic Web, with enough particles and sufficient force resolution to resolve all the galactic halos with total masses larger than 10^8 Solar masses in computational volumes of up to (163 Mpc)^3. These results were then post-processed by propagating the ionizing radiation from all sources by using fast and accurate ray-tracing radiative transfer method. Both of our codes are parallelized using a combination of MPI and OpenMP and to this date have been run efficiently on up to 2048 cores (N-body) and up to 10000 cores (radiative transfer) on the newly-deployed Sun Constellation Linux Cluster at the Texas Advanced Computing Center. In this paper we describe our codes, parallelization strategies, scaling and some preliminary scientific results. (abridged)

  • 33. Iliev, Ilian T.
    et al.
    Shapiro, Paul R.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, Department of Astronomy.
    Simulating Reionization: Character and Observability2008In: FIRST STARS III: First Stars III Conference, 2008Conference paper (Refereed)
    Abstract [en]

    In recent years there has been considerable progress in our understanding of the nature and properties of the reionization process. In particular, the numerical simulations of this epoch have made a qualitative leap forward, reaching sufficiently large scales to derive the characteristic scales of the reionization process and thus allowing for realistic observational predictions. Our group has recently performed the first such large-scale radiative transfer simulations of reionization, run on top of state-of-the-art simulations of early structure formation. This allowed us to make the first realistic observational predictions about the Epoch of Reionization based on detailed radiative transfer and structure formation simulations. We discuss the basic features of reionization derived from our simulations and some recent results on the observational implications for the high-redshift Ly-α sources.

  • 34. Jelic, V.
    et al.
    de Bruyn, A. G.
    Mevius, M.
    Abdalla, F. B.
    Asad, K. M. B.
    Bernardi, G.
    Brentjens, M. A.
    Bus, S.
    Chapman, E.
    Ciardi, B.
    Daiboo, S.
    Fernandez, E. R.
    Ghosh, A.
    Harker, G.
    Jensen, Hannes
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kazemi, S.
    Koopmans, L. V. E.
    Labropoulos, P.
    Martinez-Rubi, O.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Offringa, A. R.
    Pandey, V. N.
    Patil, A. H.
    Thomas, R. M.
    Vedantham, H. K.
    Veligatla, V.
    Yatawatta, S.
    Zaroubi, S.
    Alexov, A.
    Anderson, J.
    Avruch, I. M.
    Beck, R.
    Bell, M. E.
    Bentum, M. J.
    Best, P.
    Bonafede, A.
    Bregman, J.
    Breitling, F.
    Broderick, J.
    Brouw, W. N.
    Brueggen, M.
    Butcher, H. R.
    Conway, J. E.
    de Gasperin, F.
    de Geus, E.
    Deller, A.
    Dettmar, R. -J
    Duscha, S.
    Eisloeffel, J.
    Engels, D.
    Faleke, H.
    Fallows, R. A.
    Fender, R.
    Ferrari, C.
    Frieswijk, W.
    Garrett, M. A.
    Griessmeier, J.
    Gunst, A. W.
    Hamaker, J. P.
    Hassall, T. E.
    Haverkorn, M.
    Heald, G.
    Hessels, J. W. T.
    Hoeft, M.
    Hoerandel, J.
    Horneffer, A.
    van der Horst, A.
    Iacobelli, M.
    Juette, E.
    Karastergiou, A.
    Kondratiev, V. I.
    Kramer, M.
    Kuniyoshi, M.
    Kuper, G.
    van Leeuwen, J.
    Maat, P.
    Mann, G.
    McKay-Bakowski, D.
    MeKean, J. P.
    Mank, H.
    Nelles, A.
    Morden, M. J.
    Paas, H.
    Pandey-Pornmier, M.
    Pietka, G.
    Pizzo, R.
    Polatidis, A. G.
    Reich, W.
    Roettgering, H.
    Rowlinson, A.
    Scaife, A. M. M.
    Schwarz, D.
    Serylak, M.
    Smirnov, O.
    Steinmetz, M.
    Stewart, A.
    Tagger, M.
    Tang, Y.
    Tasse, C.
    ter Veen, S.
    Thoudam, S.
    Toribio, C.
    Vermeulen, R.
    Voeks, C.
    van Weeren, R. J.
    Wijers, R. A. M. J.
    Wijnholds, S. J.
    Wucknitz, O.
    Zarka, P.
    Initial LOFAR observations of epoch of reionization windows II. Diffuse polarized emission in the ELAIS-N1 field2014In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 568, p. A101-Article in journal (Refereed)
    Abstract [en]

    Aims. This study aims to characterise the polarized foreground emission in the ELAIS-N1 field and to address its possible implications or extracting of the cosmological 21 cm signal from the LOw-Frequency ARray - Epoch of Reionization (LOFAR-EoR) data Methods. We used the high band antennas of LOFAR to image this region and RM-synthesis to unravel structures of polarized emission at high Galactic latitudes. Results. The brightness temperature of the detected Galactic emission is on average similar to 4 K in polarized intensity and covers the range from -10 to +13 rad m(-2) in Faraday depth, The total polarized intensity and polarization angle show a wide range of morphological features. We have also used the Westerbork Synthesis Radio Telescope (WSRT) at 350 MHz to image the same region. The LOFAR and WSRT images show a similar complex morphology at comparable brightness levels, but their spatial correlation is very low. The fractional polarization at 150 MHz, expressed as a percentage of the total intensity, amounts to approximate to 1.5%. There is no indication of diffuse emission in total intensity in the interferometric data. in line with results at higher frequencies Conclusions. The wide frequency range. high angular resolution, and high sensitivity make LOFAR an exquisite instrument for studying Galactic polarized emission at a resolution of similar to 1-2 rad m(-2) in Faraday depth. The different polarized patterns observed at 150 MHz and 350 MHz are consistent with different source distributions along the line of sight wring in a variety of Faraday thin regions of emission. The presence of polarized foregrounds is a serious complication for epoch of reionization experiments. To avoid the leakage of polarized emission into total intensity, which can depend on frequency, we need to calibrate the instrumental polarization across the field of view to a small fraction of 1%.

  • 35. Jelic, Vibor
    et al.
    Zaroubi, Saleem
    Aghanim, Nabila
    Douspis, Marian
    Koopmans, Leon V. E.
    Langer, Mathieu
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy.
    Tashiro, Hiroyuki
    Thomas, Rajat M.
    A cross-correlation study between the cosmological 21 cm signal and the kinetic Sunyaev-Zel'dovich effect2010In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 402, no 4, p. 2279-2290Article in journal (Refereed)
    Abstract [en]

    The Universe's Epoch of Reionization can be studied using a number of observational probes that provide complementary or corroborating information. Each of these probes suffers from its own systematic and statistical uncertainties. It is therefore useful to consider the mutual information that these data sets contain. In this paper, we present a cross-correlation study between the kinetic Sunyaev-Zel'dovich effect - produced by the scattering of cosmic microwave background (CMB) photons off free electrons produced during the reionization process - and the cosmological 21 cm signal - which reflects the neutral hydrogen content of the Universe, as a function of redshift. The study is carried out using a simulated reionization history in 100 h(-1) Mpc scale N-body simulations with radiative transfer. In essence, we find that the two probes anticorrelate. The significance of the anticorrelation signal depends on the extent of the reionization process, wherein extended histories result in a much stronger signal compared to instantaneous cases. Unfortunately, however, once the primary CMB fluctuations are included into our simulation they serve as a source of large correlated noise that renders the cross-correlation signal insignificant, regardless of the reionization scenario.

  • 36. Jelić, V.
    et al.
    de Bruyn, A. G.
    Pandey, V. N.
    Mevius, M.
    Haverkorn, M.
    Brentjens, M. A.
    Koopmans, L. V. E.
    Zaroubi, S.
    Abdalla, F. B.
    Asad, K. M. B.
    Bus, S.
    Chapman, E.
    Ciardi, B.
    Fernandez, E. R.
    Ghosh, A.
    Harker, G.
    Iliev, I. T.
    Jensen, Hannes
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kazemi, S.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Offringa, A. R.
    Patil, A. H.
    Vedantham, H. K.
    Yatawatta, S.
    Linear polarization structures in LOFAR observations of the interstellar medium in the 3C 196 field2015In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 583, article id A137Article in journal (Refereed)
    Abstract [en]

    Aims: This study aims to characterize linear polarization structures in LOFAR observations of the interstellar medium (ISM) in the 3C 196 field, one of the primary fields of the LOFAR-Epoch of Reionization key science project.

    Methods: We have used the high band antennas (HBA) of LOFAR to image this region and rotation measure (RM) synthesis to unravel the distribution of polarized structures in Faraday depth.

    Results: The brightness temperature of the detected Galactic emission is 5-15 K in polarized intensity and covers the range from -3 to +8 rad m(-2) in Faraday depth. The most interesting morphological feature is a strikingly straight filament at a Faraday depth of +0.5 rad m(-2) running from north to south, right through the centre of the field and parallel to the Galactic plane. There is also an interesting system of linear depolarization canals conspicuous in an image showing the peaks of Faraday spectra. We used the Westerbork Synthesis Radio Telescope (WSRT) at 350 MHz to image the same region. For the first time, we see some common morphology in the RM cubes made at 150 and 350 MHz. There is no indication of diffuse emission in total intensity in the interferometric data, in line with results at higher frequencies and previous LOFAR observations. Based on our results, we determined physical parameters of the ISM and proposed a simple model that may explain the observed distribution of the intervening magneto-ionic medium.

    Conclusions: The mean line-of-sight magnetic field component, B-parallel to, is determined to be 0.3 +/- 0.1 mu G and its spatial variation across the 3C 196 field is 0.1 mu G. The filamentary structure is probably an ionized filament in the ISM, located somewhere within the Local Bubble. This filamentary structure shows an excess in thermal electron density (n(e)B(parallel to) > 6.2 cm(-3) mu G) compared to its surroundings.

  • 37.
    Jensen, Hannes
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Datta, Kanan K.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Chapman, Emma
    Abdalla, Filipe B.
    Iliev, Ilian T.
    Mao, Yi
    Santos, Mario G.
    Shapiro, Paul R.
    Zaroubi, Saleem
    Bernardi, G.
    Brentjens, M. A.
    de Bruyn, A. G.
    Ciardi, B.
    Harker, G. J. A.
    Jelic, V.
    Kazemi, S.
    Koopmans, L. V. E.
    Labropoulos, P.
    Martinez, O.
    Offringa, A. R.
    Pandey, V. N.
    Schaye, J.
    Thomas, R. M.
    Veligatla, V.
    Vedantham, H.
    Yatawatta, S.
    Probing reionization with LOFAR using 21-cm redshift space distortions2013In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 435, no 1, p. 460-474Article in journal (Refereed)
    Abstract [en]

    One of the most promising ways to study the epoch of reionization (EoR) is through radio observations of the redshifted 21-cm line emission from neutral hydrogen. These observations are complicated by the fact that the mapping of redshifts to line-of-sight positions is distorted by the peculiar velocities of the gas. Such distortions can be a source of error if they are not properly understood, but they also encode information about cosmology and astrophysics. We study the effects of redshift space distortions on the power spectrum of 21-cm radiation from the EoR using large-scale N-body and radiative transfer simulations. We quantify the anisotropy introduced in the 21-cm power spectrum by redshift space distortions and show how it evolves as reionization progresses and how it relates to the underlying physics. We go on to study the effects of redshift space distortions on LOFAR observations, taking instrument noise and foreground subtraction into account. We find that LOFAR should be able to directly observe the power spectrum anisotropy due to redshift space distortions at spatial scales around k similar to 0.1 Mpc(-1) after greater than or similar to 1000 h of integration time. At larger scales, sample errors become a limiting factor, while at smaller scales detector noise and foregrounds make the extraction of the signal problematic. Finally, we show how the astrophysical information contained in the evolution of the anisotropy of the 21-cm power spectrum can be extracted from LOFAR observations, and how it can be used to distinguish between different reionization scenarios.

  • 38.
    Jensen, Hannes
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Hayes, Matthew
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Iliev, I. T.
    Laursen, P.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Studying reionization with the next generation of Ly alpha emitter surveys2014In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 444, no 3, p. 2114-2127Article in journal (Refereed)
    Abstract [en]

    We study the prospects for constraining the ionized fraction of the intergalactic medium (IGM) at z > 6 with the next generation of large Ly alpha emitter surveys. We make predictions for the upcoming Subaru Hyper Suprime-Cam (HSC) Ly alpha survey and a hypothetical spectroscopic survey performed with the JamesWebb Space Telescope (JWST). Considering various scenarios where the observed evolution of the Ly alpha luminosity function of Ly alpha emitters at z > 6 is explained partly by an increasingly neutral IGM and partly by intrinsic galaxy evolution, we show how clustering measurements will be able to distinguish between these scenarios. We find that the HSC survey should be able to detect the additional clustering induced by a neutral IGM if the global IGM neutral fraction is greater than similar to 20 per cent at z = 6.5. If measurements of the Ly alpha equivalent widths (EWs) are also available, neutral fractions as small as 10 per cent may be detectable by looking for correlation between the EW and the local number density of objects. In this case, if it should turn out that the IGM is significantly neutral at z = 6.5 and the intrinsic EW distribution is relatively narrow, the observed EWs can also be used to construct a map of the locations and approximate sizes of the largest ionized regions. For the JWST survey, the results appear a bit less optimistic. Since such surveys probe a large range of redshifts, the effects of the IGM will be mixed up with any intrinsic galaxy evolution that is present, making it difficult to disentangle the effects. However, we show that a survey with the JWST will have a possibility of observing a large group of galaxies at z similar to 7, which would be a strong indication of a partially neutral IGM.

  • 39.
    Jensen, Hannes
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Laursen, Peter
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). University of Copenhagen, Denmark.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Iliev, Ilian T.
    Sommer-Larsen, Jesper
    Shapiro, Paul R.
    On the use of Ly alpha emitters as probes of reionization2013In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 428, no 2, p. 1366-1381Article in journal (Refereed)
    Abstract [en]

    We use numerical simulations to study the effects of the patchiness of a partly reionized intergalactic medium (IGM) on the observability of Ly alpha emitters (LAEs) at high redshifts (z greater than or similar to 6). We present a new model that divides the Ly alpha radiative transfer into a (circum) galactic and an extragalactic (IGM) part, and investigate how the choice of intrinsic line model affects the IGM transmission results. We use our model to study the impact of neutral hydrogen on statistical observables such as the Ly alpha rest-frame equivalent width (REW) distribution, the LAE luminosity function and the two-point correlation function. We find that if the observed changes in LAE luminosity functions and equivalent width distributions between z similar to 6 and 7 are to be explained by an increased IGM neutral fraction alone, we require an extremely late and rapid reionization scenario, where the Universe was similar to 40 per cent ionized at z = 7, similar to 50 per cent ionized at z = 6.5 and similar to 100 per cent ionized at z = 6. This is in conflict with other observations, suggesting that intrinsic LAE evolution at z greater than or similar to 6 cannot be completely neglected. We show how the two-point correlation function can provide more robust constraints once future observations obtain larger LAE samples, and provide predictions for the sample sizes needed to tell different reionization scenarios apart.

  • 40.
    Jensen, Hannes
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Uppsala University, Sweden.
    Majumdar, Suman
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Lidz, Adam
    Iliev, Ilian T.
    Dixon, Keri L.
    The wedge bias in reionization 21-cm power spectrum measurements2016In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 456, no 1, p. 66-70Article in journal (Refereed)
    Abstract [en]

    A proposed method for dealing with foreground emission in upcoming 21-cm observations from the epoch of reionization is to limit observations to an uncontaminated window in Fourier space. Foreground emission can be avoided in this way, since it is limited to a wedge-shaped region in k(parallel to), k(perpendicular to) space. However, the power spectrum is anisotropic owing to redshift-space distortions from peculiar velocities. Consequently, the 21-cm power spectrum measured in the foreground avoidance window - which samples only a limited range of angles close to the line-of-sight direction - differs from the full redshift-space spherically averaged power spectrum which requires an average over all angles. In this paper, we calculate the magnitude of this 'wedge bias' for the first time. We find that the bias amplifies the difference between the real-space and redshift-space power spectra. The bias is strongest at high redshifts, where measurements using foreground avoidance will overestimate the redshift-space power spectrum by around 100 per cent, possibly obscuring the distinctive rise and fall signature that is anticipated for the spherically averaged 21-cm power spectrum. In the later stages of reionization, the bias becomes negative, and smaller in magnitude (less than or similar to 20 per cent).

  • 41. Kakiichi, Koki
    et al.
    Majumdar, Suman
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Ciardi, Benedetta
    Dixon, Keri L.
    Iliev, Ilian T.
    Jelic, Vibor
    Koopmans, Leon V. E.
    Zaroubi, Saleem
    Busch, Philipp
    Recovering the H II region size statistics from 21-cm tomography2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 471, no 2, p. 1936-1954Article in journal (Refereed)
    Abstract [en]

    We introduce a novel technique, called 'granulometry', to characterize and recover the mean size and the size distribution of HII regions from 21-cm tomography. The technique is easy to implement, but places the previously not very well-defined concept of morphology on a firm mathematical foundation. The size distribution of the cold spots in 21-cm tomography can be used as a direct tracer of the underlying probability distribution of HII region sizes. We explore the capability of the method using large-scale reionization simulations and mock observational data cubes while considering capabilities of SquareKilometreArray 1 (SKA1) low and a future extension to SKA2. We show that the technique allows the recovery of the HII region size distribution with a moderate signal-to-noise ratio from wide-field imaging (SNR less than or similar to 3), for which the statistical uncertainty is sample variance dominated. We address the observational requirements on the angular resolution, the field of view, and the thermal noise limit for a successful measurement. To achieve a full scientific return from 21-cm tomography and to exploit a synergy with 21-cm power spectra, we suggest an observing strategy using widefield imaging (several tens of square degrees) by an interferometric mosaicking/multibeam observation with additional intermediate baselines (similar to 2-4 km) in an SKA phase 2.

  • 42.
    Lee, Kai Yan
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy.
    Mathematical Analysis of the Coupled Ionization and Thermal EquationsIn: Computational Astrophysics and Cosmology, ISSN 2197-7909Article in journal (Refereed)
  • 43.
    Lee, Kai Yan
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Paardekooper, Jan-Pieter
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy.
    PYRAMID - a novel ray-tracing methodIn: Computational Astrophysics and Cosmology, ISSN 2197-7909Article in journal (Refereed)
  • 44.
    Lee, Kai-Yan
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Lundqvist, Peter
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Efficient photoheating algorithms in time-dependent photoionization simulations2016In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 455, no 4, p. 4406-4425Article in journal (Refereed)
    Abstract [en]

    We present an extension to the time-dependent photoionization code C-2-RAY to calculate photoheating in an efficient and accurate way. In C-2-RAY, the thermal calculation demands relatively small time-steps for accurate results. We describe two novel methods to reduce the computational cost associated with small time-steps, namely, an adaptive time-step algorithm and an asynchronous evolution approach. The adaptive time-step algorithm determines an optimal time-step for the next computational step. It uses a fast ray-tracing scheme to quickly locate the relevant cells for this determination and only use these cells for the calculation of the time-step. Asynchronous evolution allows different cells to evolve with different time-steps. The asynchronized clocks of the cells are synchronized at the times where outputs are produced. By only evolving cells which may require short time-steps with these short time-steps instead of imposing them to the whole grid, the computational cost of the calculation can be substantially reduced. We show that our methods work well for several cosmologically relevant test problems and validate our results by comparing to the results of another time-dependent photoionization code.

  • 45.
    Majumdar, Suman
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Imperial College, UK.
    Jensen, Hannes
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Uppsala University, Sweden.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Chapman, Emma
    Abdalla, Filipe B.
    Lee, Kai-Yan
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Iliev, Ilian T.
    Dixon, Keri L.
    Datta, Kanan K.
    Ciardi, Benedetta
    Fernandez, Elizabeth R.
    Jelic, Vibor
    Koopmans, Leon V. E.
    Zaroubi, Saleem
    Effects of the sources of reionization on 21-cm redshift-space distortions2016In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 456, no 2, p. 2080-2094Article in journal (Refereed)
    Abstract [en]

    The observed 21 cm signal from the epoch of reionization will be distorted along the line of sight by the peculiar velocities of matter particles. These redshift-space distortions will affect the contrast in the signal and will also make it anisotropic. This anisotropy contains information about the cross-correlation between the matter density field and the neutral hydrogen field, and could thus potentially be used to extract information about the sources of reionization. In this paper, we study a collection of simulated reionization scenarios assuming different models for the sources of reionization. We show that the 21 cm anisotropy is best measured by the quadrupole moment of the power spectrum. We find that, unless the properties of the reionization sources are extreme in some way, the quadrupole moment evolves very predictably as a function of global neutral fraction. This predictability implies that redshift-space distortions are not a very sensitive tool for distinguishing between reionization sources. However, the quadrupole moment can be used as a model-independent probe for constraining the reionization history. We show that such measurements can be done to some extent by first-generation instruments such as LOFAR, while the SKA should be able to measure the reionization history using the quadrupole moment of the power spectrum to great accuracy.

  • 46.
    Majumdar, Suman
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Datta, Kanan K.
    Jensen, Hannes
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Choudhury, T. Roy
    Bharadwaj, Somnath
    Friedrich, Martina M.
    On the use of seminumerical simulations in predicting the 21-cm signal from the epoch of reionization2014In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 443, no 4, p. 2843-2861Article in journal (Refereed)
    Abstract [en]

    We present a detailed comparison of three different simulations of the epoch of reionization (EoR). The radiative transfer simulation (C-2-RAY) among them is our benchmark. Radiative transfer codes can produce realistic results, but are computationally expensive. We compare it with two seminumerical techniques: one using the same haloes as C-2-RAY as its sources (Sem-Num), and one using a conditional Press-Schechter scheme (CPS+GS). These are vastly more computationally efficient than C-2-RAY, but use more simplistic physical assumptions. We evaluate these simulations in terms of their ability to reproduce the history and morphology of reionization. We find that both Sem-Num and CPS+GS can produce an ionization history and morphology that is very close to C-2-RAY, with Sem-Num performing slightly better compared to CPS+GS. We also study different redshift-space observables of the 21-cm signal from EoR: the variance, power spectrum and its various angular multipole moments. We find that both seminumerical models perform reasonably well in predicting these observables at length scales relevant for present and future experiments. However, Sem-Num performs slightly better than CPS+GS in producing the reionization history, which is necessary for interpreting the future observations. The CPS+GS scheme, however, has the advantage that it is not restricted by the mass resolution of the dark matter density field.

  • 47. Mao, Yi
    et al.
    Shapiro, Paul R.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Iliev, Ilian T.
    Koda, Jun
    Ahn, Kyungjin
    Redshift-space distortion of the 21-cm background from the epoch of reionization - I. Methodology re-examined2012In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 422, no 2, p. 926-954Article in journal (Refereed)
    Abstract [en]

    The peculiar velocity of the intergalactic gas responsible for the cosmic 21-cm background from the epoch of reionization and beyond introduces an anisotropy in the three-dimensional power spectrum of brightness temperature fluctuations. Measurement of this anisotropy by future 21-cm surveys is a promising tool for separating cosmology from 21-cm astrophysics. However, previous attempts to model the signal have often neglected peculiar velocity or only approximated it crudely. This paper re-examines the effects of peculiar velocity on the 21-cm signal in detail, improving upon past treatment and addressing several issues for the first time. (1) We show that even the angle-averaged power spectrum, P(k), is affected significantly by the peculiar velocity. (2) We re-derive the brightness temperature dependence on atomic hydrogen density, spin temperature, peculiar velocity and its gradient and redshift to clarify the roles of thermal versus velocity broadening and finite optical depth. (3) We show that properly accounting for finite optical depth eliminates the unphysical divergence of the 21-cm brightness temperature in overdense regions of the intergalactic medium found by previous work that employed the usual optically thin approximation. (4) We find that the approximation made previously to circumvent the diverging brightness temperature problem by capping the velocity gradient can misestimate the power spectrum on all scales. (5) We further show that the observed power spectrum in redshift space remains finite even in the optically thin approximation if one properly accounts for the redshift-space distortion. However, results that take full account of finite optical depth show that this approximation is only accurate in the limit of high spin temperature. (6) We also show that the linear theory for redshift-space distortion widely employed to predict the 21-cm power spectrum results in a similar to 30 per cent error in the observationally relevant wavenumber range k similar to 0.1-1 h Mpc(-1), when strong ionization fluctuations exist (e. g. at the 50 per cent ionized epoch). We derive an alternative, quasi-linear formulation which improves upon the accuracy of the linear theory. (7) We describe and test two numerical schemes to calculate the 21-cm signal from reionization simulations to incorporate peculiar velocity effects in the optically thin approximation accurately, by real- to redshift-space re-mapping of the HI density. One is particle based, the other grid based, and while the former is most accurate, we demonstrate that the latter is computationally more efficient and can be optimized so as to achieve sufficient accuracy.

  • 48. Medina, S.-N. X.
    et al.
    Arthur, S. J.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Universidad Nacional Autónoma de México, México.
    Henney, W. J.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Universidad Nacional Autónoma de México, México.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Gazol, A.
    Turbulence in simulated H II regions2014In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 445, no 2, p. 1797-1819Article in journal (Refereed)
    Abstract [en]

    We investigate the scale dependence of fluctuations inside a realistic model of an evolving turbulent H II region and to what extent these may be studied observationally. We find that the multiple scales of energy injection from champagne flows and the photoionization of clumps and filaments leads to a flatter spectrum of fluctuations than would be expected from top-down turbulence driven at the largest scales. The traditional structure function approach to the observational study of velocity fluctuations is shown to be incapable of reliably determining the velocity power spectrum of our simulation. We find that a more promising approach is the Velocity Channel Analysis technique of Lazarian & Pogosyan (2000), which, despite being intrinsically limited by thermal broadening, can successfully recover the logarithmic slope of the velocity power spectrum to a precision of +/- 0.1 from high-resolution optical emission-line spectroscopy.

  • 49.
    Mellema, Garrelt
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Koopmans, Leon V. E.
    Abdalla, Filipe A.
    Bernardi, Gianni
    Ciardi, Benedetta
    Daiboo, Soobash
    de Bruyn, A. G.
    Datta, Kanan K.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Falcke, Heino
    Ferrara, Andrea
    Iliev, Ilian T.
    Iocco, Fabio
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Jelic, Vibor
    Jensen, Hannes
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Joseph, Ronniy
    Labroupoulos, Panos
    Meiksin, Avery
    Mesinger, Andrei
    Offringa, Andre R.
    Pandey, V. N.
    Pritchard, Jonathan R.
    Santos, Mario G.
    Schwarz, Dominik J.
    Semelin, Benoit
    Vedantham, Harish
    Yatawatta, Sarod
    Zaroubi, Saleem
    Reionization and the Cosmic Dawn with the Square Kilometre Array2013In: Experimental astronomy (Print), ISSN 0922-6435, E-ISSN 1572-9508, Vol. 36, no 1-2, p. 235-318Article in journal (Refereed)
    Abstract [en]

    The Square Kilometre Array (SKA) will have a low frequency component (SKA-low) which has as one of its main science goals the study of the redshifted 21 cm line from the earliest phases of star and galaxy formation in the Universe. This 21 cm signal provides a new and unique window both on the time of the formation of the first stars and accreting black holes and the subsequent period of substantial ionization of the intergalactic medium. The signal will teach us fundamental new things about the earliest phases of structure formation, cosmology and even has the potential to lead to the discovery of new physical phenomena. Here we present a white paper with an overview of the science questions that SKA-low can address, how we plan to tackle these questions and what this implies for the basic design of the telescope.

  • 50. Offringa, A. R.
    et al.
    de Bruyn, A. G.
    Zaroubi, S.
    Koopmans, L. V. E.
    Wijnholds, S. J.
    Abdalla, F. B.
    Brouw, W. N.
    Ciardi, B.
    Iliev, I. T.
    Harker, G. J. A.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy.
    Bernardi, G.
    Zarka, P.
    Ghosh, A.
    Alexov, A.
    Anderson, J.
    Asgekar, A.
    Avruch, I. M.
    Beck, R.
    Bell, M. E.
    Bell, M. R.
    Bentum, M. J.
    Best, P.
    Birzan, L.
    Breitling, F.
    Broderick, J.
    Brueggen, M.
    Butcher, H. R.
    de Gasperin, F.
    de Geus, E.
    de Vos, M.
    Duscha, S.
    Eisloeffel, J.
    Fallows, R. A.
    Ferrari, C.
    Frieswijk, W.
    Garrett, M. A.
    Griessmeier, J.
    Hassall, T. E.
    Horneffer, A.
    Iacobelli, M.
    Juette, E.
    Karastergiou, A.
    Klijn, W.
    Kondratiev, V. I.
    Kuniyoshi, M.
    Kuper, G.
    van Leeuwen, J.
    Loose, M.
    Maat, P.
    Macario, G.
    Mann, G.
    McKean, J. P.
    Meulman, H.
    Norden, M. J.
    Orru, E.
    Paas, H.
    Pandey-Pommier, M.
    Pizzo, R.
    Polatidis, A. G.
    Rafferty, D.
    Reich, W.
    van Nieuwpoort, R.
    Roettgering, H.
    Scaife, A. M. M.
    Sluman, J.
    Smirnov, O.
    Sobey, C.
    Tagger, M.
    Tang, Y.
    Tasse, C.
    ter Veen, S.
    Toribio, C.
    Vermeulen, R.
    Vocks, C.
    van Weeren, R. J.
    Wise, M. W.
    Wucknitz, O.
    The brightness and spatial distributions of terrestrial radio sources2013In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 435, no 1, p. 584-596Article in journal (Refereed)
    Abstract [en]

    Faint undetected sources of radio-frequency interference (RFI) might become visible in long radio observations when they are consistently present over time. Thereby, they might obstruct the detection of the weak astronomical signals of interest. This issue is especially important for Epoch of Reionization (EoR) projects that try to detect the faint redshifted H I signals from the time of the earliest structures in the Universe. We explore the RFI situation at 30-163 MHz by studying brightness histograms of visibility data observed with Low-Frequency Array (LOFAR), similar to radio-source-count analyses that are used in cosmology. An empirical RFI distribution model is derived that allows the simulation of RFI in radio observations. The brightness histograms show an RFI distribution that follows a power-law distribution with an estimated exponent around -1.5. With several assumptions, this can be explained with a uniform distribution of terrestrial radio sources whose radiation follows existing propagation models. Extrapolation of the power law implies that the current LOFAR EoR observations should be severely RFI limited if the strength of RFI sources remains strong after time integration. This is in contrast with actual observations, which almost reach the thermal noise and are thought not to be limited by RFI. Therefore, we conclude that it is unlikely that there are undetected RFI sources that will become visible in long observations. Consequently, there is no indication that RFI will prevent an EoR detection with LOFAR.

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