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Simulating observational probes of reionization
Stockholm University, Faculty of Science, Department of Astronomy.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The time in the history of the Universe when the first stars and galaxies formed and ionized the gas in the intergalactic medium is known as the Epoch of Reionization. This transformative time period, which took place within the first billion years after the Big Bang, is still relatively unexplored due to the significant difficulties associated with observing so far back in time. The theme of this thesis is tying together existing (mostly indirect) and upcoming observations with simulations.

Papers I and II deal with Lyα emitting galaxies. The Lyα emission line is very sensitive to neutral hydrogen, which was plentiful during the Epoch of Reionization. Therefore, observations of distant Lyα galaxies may be used to indirectly tell us something about when and how reionization took place. Properly interpreting the observations is, however, far from straightforward. In Paper I we develop a new method for combining large, low-resolution simulations of the intergalactic medium with small, high-resolution simulations of individual galaxies. We use this method to reproduce existing observations. In Paper II we use the same method to make predictions for future observations.

Another observational probe of the EoR is the 21-cm emission line from neutral hydrogen. This line is the most promising probe for directly studying the neutral gas in the early Universe, and several radio telescopes are currently gathering data to observe it. The 21-cm signal is affected by a multitude of cosmological and astrophysical effects, all of which need to be understood in order to interpret the upcoming observations. One such effect is the non-random shifts in redshifts caused by the peculiar velocity of matter flowing towards higher-density regions. This effect, known as redshift space distortions, is the topic of papers III and IV, while paper V deals with another observational effect called the lightcone effect. 

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University , 2015. , 54 p.
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-115728ISBN: 978-91-7649-143-0 (print)OAI: oai:DiVA.org:su-115728DiVA: diva2:799235
Public defence
2015-05-29, sal FB52, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 3: Manuscript.

Available from: 2015-05-07 Created: 2015-03-30 Last updated: 2016-05-18Bibliographically approved
List of papers
1. Studying reionization with the next generation of Ly alpha emitter surveys
Open this publication in new window or tab >>Studying reionization with the next generation of Ly alpha emitter surveys
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2014 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 444, no 3, 2114-2127 p.Article in journal (Refereed) Published
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.

Keyword
methods: numerical, dark ages, reionization, first stars
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-109803 (URN)10.1093/mnras/stu1600 (DOI)000343399600011 ()
Note

AuthorCount:6;

Available from: 2015-01-26 Created: 2014-12-01 Last updated: 2017-12-05Bibliographically approved
2. Light cone effect on the reionization 21-cm signal - II. Evolution, anisotropies and observational implications
Open this publication in new window or tab >>Light cone effect on the reionization 21-cm signal - II. Evolution, anisotropies and observational implications
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2014 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 442, no 2, 1491-1506 p.Article in journal (Refereed) Published
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.

Keyword
methods: numerical, methods: statistical, cosmology: theory, dark ages, reionization, first stars, diffuse radiation
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-107029 (URN)10.1093/mnras/stu927 (DOI)000339423100044 ()
Note

AuthorCount:8;

Available from: 2014-09-04 Created: 2014-09-02 Last updated: 2017-12-05Bibliographically approved
3. Effects of the sources of reionization on 21-cm redshift-space distortions
Open this publication in new window or tab >>Effects of the sources of reionization on 21-cm redshift-space distortions
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2016 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 456, no 2, 2080-2094 p.Article in journal (Refereed) Published
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.

Keyword
methods: numerical, dark ages, reionization, first stars
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-129640 (URN)10.1093/mnras/stv2812 (DOI)000372264200068 ()
Available from: 2016-04-28 Created: 2016-04-26 Last updated: 2017-11-30Bibliographically approved
4. Probing reionization with LOFAR using 21-cm redshift space distortions
Open this publication in new window or tab >>Probing reionization with LOFAR using 21-cm redshift space distortions
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2013 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 435, no 1, 460-474 p.Article in journal (Refereed) Published
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.

Keyword
instrumentation: interferometers, methods: numerical, dark ages, reionization, first stars
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-96652 (URN)10.1093/mnras/stt1341 (DOI)000325804300031 ()
Note

AuthorCount:27;

Available from: 2013-11-27 Created: 2013-11-25 Last updated: 2017-12-06Bibliographically approved
5. On the use of Ly alpha emitters as probes of reionization
Open this publication in new window or tab >>On the use of Ly alpha emitters as probes of reionization
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2013 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 428, no 2, 1366-1381 p.Article in journal (Refereed) Published
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.

Keyword
radiative transfer, methods: numerical, galaxies: high-redshift, galaxies: statistics
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-92052 (URN)10.1093/mnras/sts116 (DOI)000318229000034 ()
Funder
Swedish Research Council, 2009-4088
Note

AuthorCount:6;

Available from: 2013-07-15 Created: 2013-07-15 Last updated: 2017-12-06Bibliographically approved

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