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Tomographic studies of the 21-cm signal during reionization: Going beyond the power spectrum
Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.ORCID-id: 0000-0002-2560-536X
2019 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
Abstract [en]

The formation of the first luminous sources in the Universe, such as the first generation of stars and accreting black holes, led to the ionization of hydrogen gas present in the intergalactic medium (IGM). This period in which the Universe transitioned from a cold and neutral state to a predominantly hot and ionized state is known as the Epoch of Reionization (EoR). The EoR is one of the least understood epochs in the Universe's evolution mostly due to the lack of direct observations. We can probe the reionization process with the  21-cm signal, produced by the spin-flip transition in neutral hydrogen. However, current radio telescopes have not been able to detect this faint signal. The low-frequency component of the Square Kilometre Array (SKA-Low), will be sensitive enough not only to detect the 21-cm signal produced during EoR but also to produce images of its distribution on the sky. A sequence of such 21-cm images from different redshifts will constitute a three-dimensional, tomographic, data set. Before the SKA comes online, it is prudent to develop methods to analyse these tomographic images in a statistical sense. In this thesis, we study the prospect of understanding the EoR using such tomographic analysis methods. In Paper I, II and V, we use simulated 21-cm data sets to investigate methods to extract and interpret information from those images.  We implement a new image segmentation technique, known as superpixels, to identify ionized regions in the images and find that it performs better than previously proposed methods. Once we have identified the ionized regions (also known as bubbles), we can determine the bubble size distribution (BSD) using various size finding algorithms and use the BSDs as a summary statistics of the 21-cm signal during reionization. We also investigate the impact of different line of sight effects, such as light-cone effect and redshift space distortions on the measured BSDs. During the late stages of reionization, the BSDs become less informative since most of the IGM has become ionized. We therefore propose to study the neutral regions (also known as islands) during these late times. In Paper V, we find that most neutral islands will be relatively easy to detect with SKA-Low as they remain quite large until the end of reionization and their size distribution depends on the properties of the sources of reionization. Previous studies have shown that the 21-cm signal is highly non-Gaussian. Therefore the power spectrum cannot characterize the signal completely. In Paper III and IV, we use the bispectrum, a higher-order statistics related to the three-point correlation function, to characterize the signal. In Paper III, we probe the non-Gaussianity in the 21-cm signal caused by temperature fluctuations due to the presence of X-Ray sources. We find that the evolution of the normalized bispectrum is different from that of the power spectrum, which is useful for breaking the degeneracy between models which use different types of X-Ray sources. We also show that the 21-cm bispectrum can be constructed from observations with SKA-Low. Paper IV presents a fast and simple method to study the so-called squeezed limit version of the bispectrum, which describes how the small-scale fluctuations respond to the large-scale environment. We show that this quantity evolves during reionization and differs between different reionization scenarios.

Ort, förlag, år, upplaga, sidor
Stockholm: Department of Astronomy, Stockholm University , 2019. , s. 68
Nyckelord [en]
reionization, non-gaussianity, first stars, image processing
Nationell ämneskategori
Astronomi, astrofysik och kosmologi
Forskningsämne
astronomi
Identifikatorer
URN: urn:nbn:se:su:diva-166125ISBN: 978-91-7797-610-3 (tryckt)ISBN: 978-91-7797-611-0 (digital)OAI: oai:DiVA.org:su-166125DiVA, id: diva2:1289039
Disputation
2019-04-03, sal FB42, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (Engelska)
Opponent
Handledare
Anmärkning

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: Accepted. Paper 5: Manuscript.

Tillgänglig från: 2019-03-11 Skapad: 2019-02-15 Senast uppdaterad: 2019-11-11Bibliografiskt granskad
Delarbeten
1. Bubble size statistics during reionization from 21-cm tomography
Öppna denna publikation i ny flik eller fönster >>Bubble size statistics during reionization from 21-cm tomography
2018 (Engelska)Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 473, nr 3, s. 2949-2964Artikel i tidskrift (Refereegranskat) Published
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.

Nyckelord
methods: statistical, techniques: image processing, dark ages, reionization, first, stars, early Universe, radio lines: galaxies
Nationell ämneskategori
Astronomi, astrofysik och kosmologi
Forskningsämne
astronomi
Identifikatorer
urn:nbn:se:su:diva-153886 (URN)10.1093/mnras/stx2539 (DOI)000423809400007 ()
Tillgänglig från: 2018-03-07 Skapad: 2018-03-07 Senast uppdaterad: 2019-02-19Bibliografiskt granskad
2. Optimal identification of H II regions during reionization in 21-cm observations
Öppna denna publikation i ny flik eller fönster >>Optimal identification of H II regions during reionization in 21-cm observations
2018 (Engelska)Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 479, nr 4, s. 5596-5611Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

The ability of the future low-frequency component of the Square Kilometre Array radio telescope to produce tomographic images of the redshifted 21-cm signal will enable direct studies of the evolution of the sizes and shapes of ionized regions during the Epoch of Reionization. However, a reliable identification of ionized regions in noisy interferometric data is not trivial. Here, we introduce an image processing method known as superpixels for this purpose. We compare this method with two other previously proposed ones, one relying on a chosen threshold and the other employing automatic threshold determination using the K-Means algorithm. We use a correlation test and compare power spectra and bubble size distributions to show that the superpixels method provides a better identification of ionized regions, especially in the case of noisy data. We also describe some possible additional applications of the superpixel method, namely the derivation of the ionization history and constraints on the source properties in specific regions.

Nyckelord
methods: statistical, techniques: image processing, techniques: interferometric, dark ages, reionization, first stars, early universe, radio lines: galaxies
Nationell ämneskategori
Fysik
Identifikatorer
urn:nbn:se:su:diva-160197 (URN)10.1093/mnras/sty1786 (DOI)000441383700093 ()
Tillgänglig från: 2018-09-27 Skapad: 2018-09-27 Senast uppdaterad: 2019-02-15Bibliografiskt granskad
3. The 21-cm bispectrum as a probe of non-Gaussianities due to X-ray heating
Öppna denna publikation i ny flik eller fönster >>The 21-cm bispectrum as a probe of non-Gaussianities due to X-ray heating
Visa övriga...
2019 (Engelska)Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 482, nr 2, s. 2653-2669Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

We present analysis of the normalized 21-cm bispectrum from fully-numerical simulations of intergalactic-medium heating by stellar sources and high-mass X-ray binaries (HMXBs) during the cosmic dawn. Ly-alpha coupling is assumed to be saturated, we therefore probe the nature of non-Gaussianities produced by X-ray heating processes. We find the evolution of the normalized bispectrum to be very different from that of the power spectrum. It exhibits a turnover whose peak moves from large to small scales with decreasing redshift, and corresponds to the typical separation of emission regions. This characteristic scale reduces as more and more regions move into emission with time. Ultimately, small-scale fluctuations within heated regions come to dominate the normalized bispectrum, which at the end of the simulation is almost entirely driven by fluctuations in the density field. To establish how generic the qualitative evolution of the normalized bispectrum we see in the stellar + HMXB simulation is, we examine several other simulations - two fully numerical simulations that include quasi-stellar object (QSO) sources, and two with contrasting source properties produced with the semi-numerical simulation 21CMFAST. We find the qualitative evolution of the normalized bispectrum during X-ray heating to be generic, unless the sources of X-rays are, as with QSOs, less numerous and so exhibit more distinct isolated heated profiles. Assuming mitigation of foreground and instrumental effects are ultimately effective, we find that we should be sensitive to the normalized bispectrum during the epoch of heating, so long as the spin temperature has not saturated by z approximate to 19.

Nyckelord
methods: statistical, intergalactic medium, dark ages, reionization, first stars, cosmology: theory
Nationell ämneskategori
Astronomi, astrofysik och kosmologi
Forskningsämne
astronomi
Identifikatorer
urn:nbn:se:su:diva-165796 (URN)10.1093/mnras/sty2740 (DOI)000454578700086 ()
Tillgänglig från: 2019-02-14 Skapad: 2019-02-14 Senast uppdaterad: 2019-02-19Bibliografiskt granskad
4. Position-dependent power spectra of the 21-cm signal from the epoch of reionization
Öppna denna publikation i ny flik eller fönster >>Position-dependent power spectra of the 21-cm signal from the epoch of reionization
Visa övriga...
2019 (Engelska)Ingår i: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, nr 2, artikel-id 058Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

The 21-cm signal from the epoch of reionization is non-Gaussian. Current radio telescopes are focused on detecting the 21-cm power spectrum, but in the future the Square Kilometre Array is anticipated to provide a first measurement of the bispectrum. Previous studies have shown that the position-dependent power spectrum is a simple and efficient way to probe the squeezed-limit bispectrum. In this approach, the survey is divided into subvolumes and the correlation between the local power spectrum and the corresponding mean density of the subvolume is computed. This correlation is equivalent to an integral of the bispectrum in the squeezed limit, but is much simpler to implement than the usual bispectrum estimators. It also has a clear physical interpretation: it describes how the small-scale power spectrum of tracers such as galaxies and the 21-cm signal respond to a large-scale environment. Reionization naturally couples large and small scales as ionizing radiation produced by galactic sources can travel up to tens of Megaparsecs through the intergalactic medium during this process. Here we apply the position-dependent power spectrum approach to fluctuations in the 21-cm background from reionization. We show that this statistic has a distinctive evolution in time that can be understood with a simple analytic model. We also show that the statistic can easily distinguish between simple "inside-out" and "outside-in" models of reionization. The position-dependent power spectrum is thus a promising method to validate the reionization signal and to extract higher-order information on this process.

Nyckelord
first stars, non-gaussianity, power spectrum, reionization
Nationell ämneskategori
Astronomi, astrofysik och kosmologi
Forskningsämne
astronomi
Identifikatorer
urn:nbn:se:su:diva-166114 (URN)10.1088/1475-7516/2019/02/058 (DOI)000460061300002 ()
Tillgänglig från: 2019-02-14 Skapad: 2019-02-14 Senast uppdaterad: 2019-04-01Bibliografiskt granskad
5. Neutral island statistics during reionization from 21-cm tomography
Öppna denna publikation i ny flik eller fönster >>Neutral island statistics during reionization from 21-cm tomography
Visa övriga...
2019 (Engelska)Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 489, nr 2, s. 1590-1605Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

We present the prospects of extracting information about the epoch of reionization by identifying the remaining neutral regions, referred to as islands, in tomographic observations of the redshifted 21-cm signal. Using simulated data sets we show that at late times the 21-cm power spectrum is fairly insensitive to the details of the reionization process but that the properties of the neutral islands can distinguish between different reionization scenarios. We compare the properties of these islands with those of ionized bubbles. At equivalent volume-filling fractions, neutral islands tend to be fewer in number but larger compared to the ionized bubbles. In addition, the evolution of the size distribution of neutral islands is found to be slower than that of the ionized bubbles and also their percolation behaviour differs substantially. Even though the neutral islands are relatively rare, they will be easier to identify in observations with the low frequency component of the Square Kilometre Array due to their larger size and the lower noise levels at lower redshifts. The size distribution of neutral islands at the late stages of reionization is found to depend on the source properties, such as the ionizing efficiency of the sources and their minimum mass. We find the longest line of sight through a neutral region to be more than 100 comoving Mpc until very late stages (90-95 per cent reionized), which may have relevance for the long absorption trough at z = 5.6-5.8 in the spectrum of quasar ULAS J0148+0600.

Nyckelord
methods: statistical, techniques: image processing, techniques: interferometric, dark ages, reionization, first stars, early Universe, radio lines: galaxies
Nationell ämneskategori
Astronomi, astrofysik och kosmologi
Forskningsämne
astronomi
Identifikatorer
urn:nbn:se:su:diva-175855 (URN)10.1093/mnras/stz2224 (DOI)000489298100008 ()
Tillgänglig från: 2019-11-11 Skapad: 2019-11-11 Senast uppdaterad: 2019-11-11Bibliografiskt granskad

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