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Simulations of Cosmic Reionization: Shapes & Sizes of H II regions around Galaxies and Quasars
Stockholm University, Faculty of Science, Department of Astronomy.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

After the era of recombination, roughly 360 000 years after the big bang (redshift 1100), the universe was neutral, continued to expand and eventually the first gravitationally collapsed structures capable of forming stars, formed. Observations show that approximately 1 billion years later (redshift 6), the Universe had become highly ionized. The transition from a neutral intergalactic medium to a highly ionized one, is called the epoch of Reionization (EoR). Although quasar spectra and polarization power-spectra from cosmic microwave background experiments set some time-constrains on this epoch, the details of this process are currently not known.

New radio telescopes operating at low frequencies aim at measuring directly the neutral hydrogen content between redshifts 6 - 10 via the HI spin-flip line at 21cm. The interpretation of these first measurements is not going to be trivial. Therefore, simulations of the EoR are useful to test the many ill-constrained parameters such as the properties of the sources responsible for reionization. This thesis contributes to such simulations.

It addresses different source models and discusses different measures to quantify their effect on the shapes and sizes of the emerging H II regions. It also presents a new version of the widely used radiative transfer code C2-Ray which is capable of handling the ionizing radiation produced by energetic sources such as quasars. Using this new version we study whether 21cm experiments could detect the signature of a quasar.

We find that different size measures of ionized regions can distinguish between different source models in the simulations and that a topological measure of the ionized fraction field confirms the inside-out (i.e. overdense regions ionize first) reionization scenario. We find that the HII regions from luminous quasars may be detectable in 21cm, but that it might not be possible to distinguish them from the largest HII regions produced by clustered galaxies.

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University , 2012. , 47 p.
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-72409ISBN: 978-91-7447-448-0 (print)OAI: oai:DiVA.org:su-72409DiVA: diva2:503753
Public defence
2012-03-29, lecture room FB42, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 3: Submitted. Available from: 2012-03-07 Created: 2012-02-10 Last updated: 2012-03-05Bibliographically approved
List of papers
1. Topology and sizes of H II regions during cosmic reionization
Open this publication in new window or tab >>Topology and sizes of H II regions during cosmic reionization
Show others...
2011 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 413, no 2, 1353-1372 p.Article in journal (Refereed) Published
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.

Keyword
ISM: bubbles, H II regions, galaxies: formation, galaxies: high-redshift, intergalactic medium, cosmology: theory
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-68525 (URN)10.1111/j.1365-2966.2011.18219.x (DOI)000289904600050 ()
Note
authorCount :5Available from: 2012-01-05 Created: 2012-01-04 Last updated: 2017-12-08Bibliographically approved
2. Radiative Transfer of energetic photons: X-rays and helium ionization in C2-Ray
Open this publication in new window or tab >>Radiative Transfer of energetic photons: X-rays and helium ionization in C2-Ray
2012 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 421, no 3, 2232-2250 p.Article in journal (Refereed) Published
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.

Keyword
radiative transfer, methods: numerical, H ii regions, intergalactic medium
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-74004 (URN)10.1111/j.1365-2966.2012.20449.x (DOI)000302620500030 ()
Available from: 2012-02-27 Created: 2012-02-27 Last updated: 2017-12-07Bibliographically approved
3. Prospects of observing a quasar HII region during the Epoch ofReionization with redshifted 21cm
Open this publication in new window or tab >>Prospects of observing a quasar HII region during the Epoch ofReionization with redshifted 21cm
(English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966Article in journal (Refereed) Submitted
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.

National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-74229 (URN)
Available from: 2012-03-02 Created: 2012-03-02 Last updated: 2017-12-07Bibliographically approved

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