Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Neutral island statistics during reionization from 21-cm tomography
Stockholm University, Faculty of Science, Department of Astronomy.ORCID iD: 0000-0002-2560-536X
Stockholm University, Faculty of Science, Department of Astronomy.ORCID iD: 0000-0002-2512-6748
Stockholm University, Faculty of Science, Department of Astronomy.
Show others and affiliations
(English)Manuscript (preprint) (Other academic)
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 (SKA-Low) 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 megaparsecs 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 ULASJ0148+0600.

Keywords [en]
dark ages, reionization, first stars – early universe – methods: statistical – radio lines: galaxies – techniques: interferometric – techniques: image processing
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-166123OAI: oai:DiVA.org:su-166123DiVA, id: diva2:1288898
Available from: 2019-02-14 Created: 2019-02-14 Last updated: 2019-02-18Bibliographically 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)
Opponent
Supervisors
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: 2019-03-01Bibliographically approved

Open Access in DiVA

No full text in DiVA

Search in DiVA

By author/editor
Giri, Sambit K.Mellema, Garrelt
By organisation
Department of Astronomy
Astronomy, Astrophysics and Cosmology

Search outside of DiVA

GoogleGoogle Scholar

urn-nbn

Altmetric score

urn-nbn
Total: 11 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf