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The 21-cm bispectrum as a probe of non-Gaussianities due to X-ray heating
Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). University of Sussex, UK.
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Number of Authors: 72019 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 482, no 2, p. 2653-2669Article in journal (Refereed) 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.

Place, publisher, year, edition, pages
2019. Vol. 482, no 2, p. 2653-2669
Keywords [en]
methods: statistical, intergalactic medium, dark ages, reionization, first stars, cosmology: theory
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-165796DOI: 10.1093/mnras/sty2740ISI: 000454578700086OAI: oai:DiVA.org:su-165796DiVA, id: diva2:1288683
Available from: 2019-02-14 Created: 2019-02-14 Last updated: 2019-02-19Bibliographically approved
In thesis
1. Tomographic studies of the 21-cm signal during reionization: Going beyond the power spectrum
Open this publication in new window or tab >>Tomographic studies of the 21-cm signal during reionization: Going beyond the power spectrum
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
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.

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University, 2019. p. 68
Keywords
reionization, non-gaussianity, first stars, image processing
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-166125 (URN)978-91-7797-610-3 (ISBN)978-91-7797-611-0 (ISBN)
Public defence
2019-04-03, sal FB42, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
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Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: Accepted.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

Available from: 2019-03-11 Created: 2019-02-15 Last updated: 2019-11-11Bibliographically approved

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Giri, Sambit K.Ross, Hannah E.Mellama, Garrelt
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