We describe a series of reionization and galaxy formation simulations performed at HLRS within th... more We describe a series of reionization and galaxy formation simulations performed at HLRS within the “Small Scale Structure of the Universe” (SSSU) project.
When the first galaxies formed and starlight escaped into the intergalactic medium to reionize it... more When the first galaxies formed and starlight escaped into the intergalactic medium to reionize it, galaxy formation and reionization were both highly inhomogeneous in time and space, and fully-coupled by mutual feedback. To show how this imprinted the UV luminosity function (UVLF) of reionization-era galaxies, we use our large-scale, radiation-hydrodynamics simulation CoDa II to derive the time-and space-varying halo mass function and UVLF, from ≃ 6-15. That UVLF correlates strongly with local reionization redshift: earlier-reionizing regions have UVLFs that are higher, more extended to brighter magnitudes, and flatter at the faint end than later-reionizing regions observed at the same. In general, as a region reionizes, the faint-end slope of its local UVLF flattens, and, by = 6 (when reionization ended), the global UVLF, too, exhibits a flattened faint-end slope, 'rolling-over' at UV ≳ −17. CoDa II's UVLF is broadly consistent with cluster-lensed galaxy observations of the Hubble Frontier Fields at = 6-8, including the faint end, except for the faintest data point at = 6, based on one galaxy at UV = −12.5. According to CoDa II, the probability of observing the latter is ∼ 5%. However, the effective volume searched at this magnitude is very small, and is thus subject to significant cosmic variance. We find that previous methods adopted to calculate the uncertainty due to cosmic variance underestimated it on such small scales by a factor of 2-4, primarily by underestimating the variance in halo abundance when the sample volume is small.
Proceedings of the International Astronomical Union, Jun 1, 2014
We examine the stacked integrated Sachs-Wolfe (ISW) imprints on the CMB along the lines of sight ... more We examine the stacked integrated Sachs-Wolfe (ISW) imprints on the CMB along the lines of sight of voids and superclusters in galaxy surveys, using the Jubilee ISW simulation and mock luminous red galaxy (LRG) catalogues. We show that the expected signal in the concordance ΛCDM model is much smaller than the primary anisotropies arising at the last scattering surface and therefore any currently claimed detections of such an imprint cannot be caused by the ISW effect in ΛCDM. We look for the existence of such a signal in the Planck CMB using a catalogue of voids and superclusters from the Sloan Digital Sky Survey (SDSS), but find a result completely consistent with ΛCDM-i.e., a null detection.
Monthly Notices of the Royal Astronomical Society, Jul 9, 2021
The 21-cm signal from the Cosmic Dawn (CD) is likely to contain large fluctuations, with the most... more The 21-cm signal from the Cosmic Dawn (CD) is likely to contain large fluctuations, with the most extreme astrophysical models on the verge of being ruled out by observations from radio interferometers. It is therefore vital that we understand not only the astrophysical processes go v erning this signal, but also other inherent processes impacting the signal itself, and in particular line-of-sight effects. Using our suite of fully numerical radiative transfer simulations, we investigate the impact on the redshifted 21-cm from the CD from one of these processes, namely the redshift-space distortions (RSDs). When RSDs are added, the resulting boost to the power spectra makes the signal more or equally detectable for our models for all redshifts, further strengthening hopes that a power spectra measurement of the CD will be possible. RSDs lead to anisotropy in the signal at the beginning and end of the CD, but not while X-ray heating is underway. The inclusion of RSDs, ho we ver, decreases detectability of the non-Gaussianity of fluctuations from inhomogeneous X-ray heating as measured by the skewness and kurtosis. On the other hand, mock observations created from all our simulations that include telescope noise corresponding to 1000 h of observation with the Square Kilometre Array telescope show that we may be able to image the CD for all heating models considered and suggest RSDs dramatically boost fluctuations coming from the inhomogeneous Ly α background.
Monthly Notices of the Royal Astronomical Society, Aug 19, 2016
We propose a novel technique to separate the late-time, post-reionization component of the kineti... more We propose a novel technique to separate the late-time, post-reionization component of the kinetic Sunyaev-Zeldovich (kSZ) effect from the contribution to it from a (poorly understood and probably patchy) reionization history. The kSZ effect is one of the most promising probe of the missing baryons in the Universe. We study the possibility of reconstructing it in three dimensions (3D), using future spectroscopic surveys such as the Euclid survey. Unlike the thermal Sunyaev-Zel'dovich (tSZ) effect, direct cross-correlation of kSZ effect with projected galaxy distribution suffers from cancellation due to the vector (or spin-1) nature of the kSZ effect. By reconstructing a 3D template from galaxy density and peculiar velocity fields from spectroscopic surveys we cross-correlate the estimator against CMB maps. The resulting cross-correlation can help us to map out the kSZ contribution to CMB in 3D as a function of redshift thereby extending previous results which use tomographic reconstruction. This allows the separation of the late time effect from the contribution owing to reionization. By construction, it avoids contamination from foregrounds, primary CMB, tSZ effect as well as from star forming galaxies. Due to a high number density of galaxies the signal-to-noise (S/N) for such cross-correlational studies are higher, compared to the studies involving CMB power spectrum analysis. Using a spherical Bessel-Fourier (sFB) transform we introduce a pair of 3D power-spectra: C ℓ (k) and C ⊥ ℓ (k) that can be used for this purpose. We find that in a future spectroscopic survey with near all-sky coverage and a survey depth of z ≈ 1, reconstruction of C ⊥ ℓ (k) can be achieved in a few radial wave bands k ≈ (0.01 − 0.5h −1 Mpc) with a S/N of upto O(10) for angular harmonics in the range ℓ = (200 − 2000).
The emissions from the first luminous sources drive the fluctuations in the 21cm signal at Cosmic... more The emissions from the first luminous sources drive the fluctuations in the 21cm signal at Cosmic Dawn (CD) via two main astrophysical processes, namely Ly coupling and X-ray heating, yielding a highly non-Gaussian signal. The impact of these processes on the 21-cm signal and its non-Gaussianity depends on the properties of these first sources of light. In this work, we consider different CD scenarios by varying two major source parameters i.e. the minimum halo mass h, min and X-ray photon production efficiency X using the 1D radiative transfer code. We study the impact of variation in these source parameters on the large scale (1 = 0.16 Mpc −1) 21-cm bispectrum for all possible unique triangles in the Fourier domain. Our detailed and comparative analysis of the power spectrum and bispectrum shows that the shape, sign and magnitude of the bispectrum jointly provide a better measure of the signal fluctuations and its non-Gaussianity than the power spectrum alone. We also conclude that it is important to study the sequence of sign changes along with the variations in the shape and magnitude of the bispectrum throughout the CD history to arrive at a robust conclusion about the dominant processes in the intergalactic medium at different cosmic times. We further observe that among all the possible uniquetriangles, the large-scale non-Gaussianity of the signal is best probed by the small-triangles in the squeezed limit and by triangle shapes in its vicinity. This opens up the possibility of constraining the source parameters during the CD using the 21-cm bispectrum.
Journal of Cosmology and Astroparticle Physics, Nov 1, 2022
The emissions from the first luminous sources drive the fluctuations in the 21cm signal at Cosmic... more The emissions from the first luminous sources drive the fluctuations in the 21cm signal at Cosmic Dawn (CD) via two main astrophysical processes, namely Ly coupling and X-ray heating, yielding a highly non-Gaussian signal. The impact of these processes on the 21-cm signal and its non-Gaussianity depends on the properties of these first sources of light. In this work, we consider different CD scenarios by varying two major source parameters i.e. the minimum halo mass h, min and X-ray photon production efficiency X using the 1D radiative transfer code. We study the impact of variation in these source parameters on the large scale (1 = 0.16 Mpc −1) 21-cm bispectrum for all possible unique triangles in the Fourier domain. Our detailed and comparative analysis of the power spectrum and bispectrum shows that the shape, sign and magnitude of the bispectrum jointly provide a better measure of the signal fluctuations and its non-Gaussianity than the power spectrum alone. We also conclude that it is important to study the sequence of sign changes along with the variations in the shape and magnitude of the bispectrum throughout the CD history to arrive at a robust conclusion about the dominant processes in the intergalactic medium at different cosmic times. We further observe that among all the possible uniquetriangles, the large-scale non-Gaussianity of the signal is best probed by the small-triangles in the squeezed limit and by triangle shapes in its vicinity. This opens up the possibility of constraining the source parameters during the CD using the 21-cm bispectrum.
Monthly Notices of the Royal Astronomical Society, May 4, 2019
The upcoming radio interferometer Square Kilometre Array (SKA) is expected to directly detect the... more The upcoming radio interferometer Square Kilometre Array (SKA) is expected to directly detect the redshifted 21-cm signal from the neutral hydrogen present during the Cosmic Dawn. Temperature fluctuations from X-ray heating of the neutral intergalactic medium can dominate the fluctuations in the 21-cm signal from this time. This heating depends on the abundance, clustering, and properties of the X-ray sources present, which remain highly uncertain. We present a suite of three new large-volume, 349 Mpc a side, fully numerical radiative transfer simulations including QSO-like sources, extending the work previously presented in Ross et al. (2017). The results show that our QSOs have a modest contribution to the heating budget, yet significantly impact the 21-cm signal. Initially, the power spectrum is boosted on large scales by heating from the biased QSO-like sources, before decreasing on all scales. Fluctuations from images of the 21-cm signal with resolutions corresponding to SKA1-Low at the appropriate redshifts are well above the expected noise for deep integrations, indicating that imaging could be feasible for all the X-ray source models considered. The most notable contribution of the QSOs is a dramatic increase in non-Gaussianity of the signal, as measured by the skewness and kurtosis of the 21-cm probability distribution functions. However, in the case of late Lyman-α saturation, this non-Gaussianity could be dramatically decreased particularly when heating occurs earlier. We conclude that increased non-Gaussianity is a promising signature of rare X-ray sources at this time, provided that Lyman-α saturation occurs before heating dominates the 21-cm signal.
A number of large current experiments aim to detect the signatures of the Cosmic Reionization at ... more A number of large current experiments aim to detect the signatures of the Cosmic Reionization at redshifts z > 6. Their success depends crucially on understanding the character of the reionization process and its observable consequences and designing the best strategies to use. We use large-scale simulations of cosmic reionization to evaluate the reionization signatures at redshifted 21-cm and small-scale CMB anisotropies in the best current model for the background universe, with fundamental cosmological parameters given by WMAP 3year results (WMAP3). We find that the optimal frequency range for observing the "global step" of the 21-cm emission is 120-150 MHz, while statistical studies should aim at 140-160 MHz, observable by GMRT. Some strongly-nongaussian brightness features should be detectable at frequencies up to ∼ 190 MHz. In terms of sensitivity-signal trade-off relatively low resolutions, corresponding to beams of at least a few arcminutes, are preferable. The CMB anisotropy signal from the kinetic Sunyaev-Zel'dovich effect from reionized patches peaks at tens of µK at arcminute scales and has an rms of ∼ 1µK, and should be observable by the Atacama Cosmology Telescope and the South Pole Telescope. We discuss the various observational issues and the uncertainties involved, mostly related to the poorly-known reionization parameters and, to a lesser extend, to the uncertainties in the background cosmology.
A novel method to characterize the topology of the early-universe intergalactic medium during the... more A novel method to characterize the topology of the early-universe intergalactic medium during the epoch of cosmic reionization is presented. The 21-cm radiation background from high redshift is analyzed through the calculation of the 2-dimensional (2D) genus. The radiative transfer of hydrogen-ionizing photons and ionization-rate equations are calculated in a suite of numerical simulations under various input parameters. The 2D genus is calculated from the mock 21-cm images of the high-redshift Universe. We construct the 2D genus curve by varying the threshold differential brightness temperature, and compare this to the 2D genus curve of the underlying density field. We find that (1) the 2D genus curve reflects the evolutionary track of cosmic reionization and (2) the 2D genus curve can discriminate between certain reionization scenarios and thus indirectly probe the properties of radiation-sources. Choosing the right beam shape of a radio antenna is crucial for this analysis. To this end, the Square Kilometre Array (SKA) is found to be a suitable apparatus for this analysis in terms of sensitivity, even though some deterioration of the data for this purpose is unavoidable under the planned size of the antenna core.
Monthly Notices of the Royal Astronomical Society, Apr 3, 2013
We present detailed radiative transfer simulations of the reionization of the Milky Way by metal-... more We present detailed radiative transfer simulations of the reionization of the Milky Way by metal-poor globular clusters. We identify potential metal-poor globular cluster candidates within the Aquarius simulation using dark matter halo velocity dispersions. We calculate the local ionization fields via a photon-conserving, three dimensional non-equilibrium chemistry code. The key feature of the model is that globular cluster formation is suppressed if the local gas is ionized. We assume that at these early times, the ionization field is dominated by the flux from metal-poor globular clusters. Our spatial treatment of the ionization field leads to drastically different numbers and spatial distributions when compared to models where globular cluster formation is simply truncated at early redshifts (z ∼ 13). The spatial distributions are more extended and more globular clusters are produced. We find that additional sources of ionization are required at later epochs (z ∼ 10) to ionize the remaining gas and recover radial distributions statistically consistent with that of the Milky Way metal-poor globular clusters. We investigate a range of plausible ionization efficiencies to determine the effect photon-rich and photon-poor models have on present-day globular cluster properties. If globular clusters do indeed form within high-redshift dark matter haloes, they produce enough photons to ionize 98 and 90 per cent local (i.e. 2 3 h −3 Mpc 3 centred on the host galaxy) volume and mass by redshift 10, respectively. In our photon-poorest model, this contribution drops to 60 and 50 per cent. Our model therefore implies that globular clusters are important contributors to the reionization process on local scales at high-redshift until more photon-rich sources dominate the photon budget at later times. The surviving clusters in all models have a narrow average age range (mean = 13.34 Gyr, σ = 0.04 Gyr) consistent with current age estimates of the Milky Way metal-poor globular clusters. We also test a simple dynamical destruction model and estimate that ∼60 per cent of all metal-poor globular clusters formed at high redshift have since been destroyed via tidal interactions with the host galaxy.
Monthly Notices of the Royal Astronomical Society, Sep 17, 2007
The reionization of the Universe has profound effects on the way galaxies form and on their obser... more The reionization of the Universe has profound effects on the way galaxies form and on their observed properties at later times. Of particular importance is the relative timing of the reionization history of a region and its halo assembly history, which can affect the nature of the first stars formed in that region, the properties and radial distribution of its stellar halo, globular cluster population and its satellite galaxies. We distinguish two basic cases for the reionization of a halo-internal reionization, whereby the stars forming in situ reionize their host galaxy, and external reionization, whereby the progenitor of a galaxy is reionized by external radiation before its own stars are able to form in sufficient numbers. We use a set of large-scale radiative transfer and structure formation simulations, based on cosmologies derived from both Wilkinson Microwave Anisotropy Probe (WMAP) one-year and WMAP three-year data, to evaluate the mean reionization redshifts and the probability of internal/external reionization for Local Group-like systems, galaxies in the field and central cD galaxies in clusters. We find that these probabilities are strongly dependent on the underlying cosmology and the efficiency of photon production, but also on the halo mass. There is a rapid transition between predominantly external and predominantly internal reionization at a mass scale of ∼10 12 M (corresponding roughly to L * galaxies), with haloes less massive than this being reionized preferentially from distant sources. We provide a fit for the reionization redshift as a function of halo mass, which could be helpful to parametrize reionization in semi-analytical models of galaxy formation on cosmological scales. We find no statistical correlation between the reionization history of field galaxies and their environment.
The prospect of detecting the first galaxies by observing their impact on the intergalactic mediu... more The prospect of detecting the first galaxies by observing their impact on the intergalactic medium (IGM) as they reionized it during the first billion years leads us to ask whether such indirect observations are capable of diagnosing which types of galaxies were most responsible for reionization. We attempt to answer this by extending the galaxy mass range of our first generation of large-scale radiative transfer simulations of reionization in volumes large enough, (100/hM pc) 3 , to make statistically meaningful predictions of observable signatures, downward from those above 10 9 M ⊙ (high-mass, atomic-cooling halos, or "HMACHs") to include those between 10 8 and 10 9 M ⊙ (low-mass, atomic-cooling halos, or "LMACHs"), as well. Previously, we simulated the effects of both HMACHs and LMACHs but only by reducing the box size to 35/h Mpc, too small to apply those simulations to predict observables like the 21-cm background fluctuations. Those simulations showed that LMACHS can make a difference, however. While LMACHs are even more abundant, photoheating suppresses their ability to form stars if they are located inside ionized regions of the IGM, so their contribution starts reionization earlier but tends to saturate over time before it ends. To predict the observables in that case while explicitly tracking the radiation from this full mass range of galaxies, we have had to advance both our N-body and radiative transfer methods substantially, as described here. With this, we perform new simulations in a box 163 Mpc on a side, and focus here on predictions of the 21cm background, to see if upcoming observations are capable of distinguishing a universe ionized primarily by HMACHS from one in which both HMACHs and LMACHs are responsible, and to see how these results depend upon the uncertain source efficiencies. We find that 21-cm fluctuation power spectra observed by the first generation EoR/21cm radio interferometer arrays should be able to distinguish the case of reionization by HMACHs alone from that by both HMACHs and LMACHs, together. Some reionization scenarios, e.g. one with abundant low-efficiency sources vs. one with self-regulation, yield very similar power spectra and rms evolution and thus can only be discriminated by their different mean reionization history and 21-cm PDF distributions. We find that the skewness of the 21-cm PDF distribution smoothed over LOFAR-like window shows a clear feature correlated with the rise of the rms due to patchiness. This is independent on the reionization scenario and thus provides a new approach for detecting the rise of large-scale patchiness and an independent check on other measurements, regardless of the detailed properties of the sources. The peak epoch of the 21-cm rms fluctuations depends significantly on the beam and bandwidth smoothing size as well as on the reionization scenario and can occur for ionized fractions as low as 30% and as high as 70%. Measurements of the mean photoionization rates are sensitive to the average density of the regions being studied and therefore could be strongly skewed in certain cases. Finally, the simulation volume employed has very modest effects on the results during the early and intermediate stages of reionization, but late-time signatures could be significantly affected.
Proceedings of Advancing Astrophysics with the Square Kilometre Array — PoS(AASKA14), 2015
The local intensity of the 21 cm signal emitted during the Epoch of Reionization that will be map... more The local intensity of the 21 cm signal emitted during the Epoch of Reionization that will be mapped by the SKA is modulated by the amount of neutral hydrogen. Consequently, understanding the process of reionization of the intergalactic medium (IGM) is crucial for predicting and interpreting the upcoming observations. After presenting the basic physics and most meaningful quantities pertaining to the process of reionization, we will review recent progress in our understanding of the production and escape of ionizing photons in primordial galaxies and of their absorption in the IGM especially in so-called minihalos and Lyman Limit Systems.
Monthly Notices of the Royal Astronomical Society, 2020
We derive constraints on the thermal and ionization states of the intergalactic medium (IGM) at r... more We derive constraints on the thermal and ionization states of the intergalactic medium (IGM) at redshift ≈ 9.1 using new upper limits on the 21-cm power spectrum measured by the LOFAR radio telescope and a prior on the ionized fraction at that redshift estimated from recent cosmic microwave background (CMB) observations. We have used results from the reionization simulation code grizzly and a Bayesian inference framework to constrain the parameters which describe the physical state of the IGM. We find that, if the gas heating remains negligible, an IGM with ionized fraction ≳0.13 and a distribution of the ionized regions with a characteristic size ≳ 8 h−1 comoving megaparsec (Mpc) and a full width at half-maximum (FWHM) ≳16 h−1 Mpc is ruled out. For an IGM with a uniform spin temperature TS ≳ 3 K, no constraints on the ionized component can be computed. If the large-scale fluctuations of the signal are driven by spin temperature fluctuations, an IGM with a volume fraction ≲0.34 of h...
Monthly Notices of the Royal Astronomical Society, 2019
The upcoming radio interferometer Square Kilometre Array (SKA) is expected to directly detect the... more The upcoming radio interferometer Square Kilometre Array (SKA) is expected to directly detect the redshifted 21-cm signal from the neutral hydrogen present during the Cosmic Dawn. Temperature fluctuations from X-ray heating of the neutral intergalactic medium can dominate the fluctuations in the 21-cm signal from this time. This heating depends on the abundance, clustering, and properties of the X-ray sources present, which remain highly uncertain. We present a suite of three new large-volume, 349 Mpc a side, fully numerical radiative transfer simulations including QSO-like sources, extending the work previously presented in Ross et al. (2017). The results show that our QSOs have a modest contribution to the heating budget, yet significantly impact the 21-cm signal. Initially, the power spectrum is boosted on large scales by heating from the biased QSO-like sources, before decreasing on all scales. Fluctuations from images of the 21-cm signal with resolutions corresponding to SKA1-Low at the appropriate redshifts are well above the expected noise for deep integrations, indicating that imaging could be feasible for all the X-ray source models considered. The most notable contribution of the QSOs is a dramatic increase in non-Gaussianity of the signal, as measured by the skewness and kurtosis of the 21-cm probability distribution functions. However, in the case of late Lyman-α saturation, this non-Gaussianity could be dramatically decreased particularly when heating occurs earlier. We conclude that increased non-Gaussianity is a promising signature of rare X-ray sources at this time, provided that Lyman-α saturation occurs before heating dominates the 21-cm signal.
Monthly Notices of the Royal Astronomical Society, 2014
We study the prospects for constraining the ionized fraction of the intergalactic medium (IGM) at... more We study the prospects for constraining the ionized fraction of the intergalactic medium (IGM) at z > 6 with the next generation of large Lyα emitter surveys. We make predictions for the upcoming Subaru Hyper Suprime-Cam (HSC) Lyα survey and a hypothetical spectroscopic survey performed with the James Webb Space Telescope (JWST). Considering various scenarios where the observed evolution of the Lyα luminosity function of Lyα 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 ∼20 per cent at z = 6.5. If measurements of the Lyα 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 ∼ 7, which would be a strong indication of a partially neutral IGM.
Monthly Notices of the Royal Astronomical Society, 2015
We present estimates of the non-linear bias of cosmological halo formation, spanning a wide range... more We present estimates of the non-linear bias of cosmological halo formation, spanning a wide range in the halo mass from ∼10 5 to ∼10 12 M , based upon both a suite of high-resolution cosmological N-body simulations and theoretical predictions. The halo bias is expressed in terms of the mean bias and stochasticity as a function of local overdensity (δ), under different filtering scales, which is realized as the density of individual cells in uniform grids. The sampled overdensities span a range wide enough to provide the fully non-linear bias effect on the formation of haloes. A strong correlation between δ and halo population overdensity δ h is found, along with sizable stochasticity. We find that the empirical mean halo bias matches, with good accuracy, the prediction by the peak-background split method based on the excursion set formalism, as long as the empirical, globally averaged halo mass function is used. Consequently, this bias formalism is insensitive to uncertainties caused by varying halo-identification schemes, and can be applied generically. We also find that the probability distribution function of biased halo numbers has wider distribution than the pure Poisson shot noise, which is attributed to the sub-cell-scale halo correlation. We explicitly calculate this correlation function and show that both overdense and underdense regions have positive correlation, leading to stochasticity larger than the Poisson shot noise in the range of haloes and halo-collapse epochs we study.
In recent years there has been considerable progress in our understanding of the nature and prope... more In recent years there has been considerable progress in our understanding of the nature and properties of the reionization process. In particular, the numerical simulations of this epoch have made a qualitative leap forward, reaching sufficiently large scales to derive the characteristic scales of the reionization process and thus allowing for realistic observational predictions. Our group has recently performed the first such large-scale radiative transfer simulations of reionization, run on top of state-of-the-art simulations of early structure formation. This allowed us to make the first realistic observational predictions about the Epoch of Reionization based on detailed radiative transfer and structure formation simulations. We discuss the basic features of reionization derived from our simulations and some recent results on the observational implications for the high-redshift Ly-α sources.
We describe a series of reionization and galaxy formation simulations performed at HLRS within th... more We describe a series of reionization and galaxy formation simulations performed at HLRS within the “Small Scale Structure of the Universe” (SSSU) project.
When the first galaxies formed and starlight escaped into the intergalactic medium to reionize it... more When the first galaxies formed and starlight escaped into the intergalactic medium to reionize it, galaxy formation and reionization were both highly inhomogeneous in time and space, and fully-coupled by mutual feedback. To show how this imprinted the UV luminosity function (UVLF) of reionization-era galaxies, we use our large-scale, radiation-hydrodynamics simulation CoDa II to derive the time-and space-varying halo mass function and UVLF, from ≃ 6-15. That UVLF correlates strongly with local reionization redshift: earlier-reionizing regions have UVLFs that are higher, more extended to brighter magnitudes, and flatter at the faint end than later-reionizing regions observed at the same. In general, as a region reionizes, the faint-end slope of its local UVLF flattens, and, by = 6 (when reionization ended), the global UVLF, too, exhibits a flattened faint-end slope, 'rolling-over' at UV ≳ −17. CoDa II's UVLF is broadly consistent with cluster-lensed galaxy observations of the Hubble Frontier Fields at = 6-8, including the faint end, except for the faintest data point at = 6, based on one galaxy at UV = −12.5. According to CoDa II, the probability of observing the latter is ∼ 5%. However, the effective volume searched at this magnitude is very small, and is thus subject to significant cosmic variance. We find that previous methods adopted to calculate the uncertainty due to cosmic variance underestimated it on such small scales by a factor of 2-4, primarily by underestimating the variance in halo abundance when the sample volume is small.
Proceedings of the International Astronomical Union, Jun 1, 2014
We examine the stacked integrated Sachs-Wolfe (ISW) imprints on the CMB along the lines of sight ... more We examine the stacked integrated Sachs-Wolfe (ISW) imprints on the CMB along the lines of sight of voids and superclusters in galaxy surveys, using the Jubilee ISW simulation and mock luminous red galaxy (LRG) catalogues. We show that the expected signal in the concordance ΛCDM model is much smaller than the primary anisotropies arising at the last scattering surface and therefore any currently claimed detections of such an imprint cannot be caused by the ISW effect in ΛCDM. We look for the existence of such a signal in the Planck CMB using a catalogue of voids and superclusters from the Sloan Digital Sky Survey (SDSS), but find a result completely consistent with ΛCDM-i.e., a null detection.
Monthly Notices of the Royal Astronomical Society, Jul 9, 2021
The 21-cm signal from the Cosmic Dawn (CD) is likely to contain large fluctuations, with the most... more The 21-cm signal from the Cosmic Dawn (CD) is likely to contain large fluctuations, with the most extreme astrophysical models on the verge of being ruled out by observations from radio interferometers. It is therefore vital that we understand not only the astrophysical processes go v erning this signal, but also other inherent processes impacting the signal itself, and in particular line-of-sight effects. Using our suite of fully numerical radiative transfer simulations, we investigate the impact on the redshifted 21-cm from the CD from one of these processes, namely the redshift-space distortions (RSDs). When RSDs are added, the resulting boost to the power spectra makes the signal more or equally detectable for our models for all redshifts, further strengthening hopes that a power spectra measurement of the CD will be possible. RSDs lead to anisotropy in the signal at the beginning and end of the CD, but not while X-ray heating is underway. The inclusion of RSDs, ho we ver, decreases detectability of the non-Gaussianity of fluctuations from inhomogeneous X-ray heating as measured by the skewness and kurtosis. On the other hand, mock observations created from all our simulations that include telescope noise corresponding to 1000 h of observation with the Square Kilometre Array telescope show that we may be able to image the CD for all heating models considered and suggest RSDs dramatically boost fluctuations coming from the inhomogeneous Ly α background.
Monthly Notices of the Royal Astronomical Society, Aug 19, 2016
We propose a novel technique to separate the late-time, post-reionization component of the kineti... more We propose a novel technique to separate the late-time, post-reionization component of the kinetic Sunyaev-Zeldovich (kSZ) effect from the contribution to it from a (poorly understood and probably patchy) reionization history. The kSZ effect is one of the most promising probe of the missing baryons in the Universe. We study the possibility of reconstructing it in three dimensions (3D), using future spectroscopic surveys such as the Euclid survey. Unlike the thermal Sunyaev-Zel'dovich (tSZ) effect, direct cross-correlation of kSZ effect with projected galaxy distribution suffers from cancellation due to the vector (or spin-1) nature of the kSZ effect. By reconstructing a 3D template from galaxy density and peculiar velocity fields from spectroscopic surveys we cross-correlate the estimator against CMB maps. The resulting cross-correlation can help us to map out the kSZ contribution to CMB in 3D as a function of redshift thereby extending previous results which use tomographic reconstruction. This allows the separation of the late time effect from the contribution owing to reionization. By construction, it avoids contamination from foregrounds, primary CMB, tSZ effect as well as from star forming galaxies. Due to a high number density of galaxies the signal-to-noise (S/N) for such cross-correlational studies are higher, compared to the studies involving CMB power spectrum analysis. Using a spherical Bessel-Fourier (sFB) transform we introduce a pair of 3D power-spectra: C ℓ (k) and C ⊥ ℓ (k) that can be used for this purpose. We find that in a future spectroscopic survey with near all-sky coverage and a survey depth of z ≈ 1, reconstruction of C ⊥ ℓ (k) can be achieved in a few radial wave bands k ≈ (0.01 − 0.5h −1 Mpc) with a S/N of upto O(10) for angular harmonics in the range ℓ = (200 − 2000).
The emissions from the first luminous sources drive the fluctuations in the 21cm signal at Cosmic... more The emissions from the first luminous sources drive the fluctuations in the 21cm signal at Cosmic Dawn (CD) via two main astrophysical processes, namely Ly coupling and X-ray heating, yielding a highly non-Gaussian signal. The impact of these processes on the 21-cm signal and its non-Gaussianity depends on the properties of these first sources of light. In this work, we consider different CD scenarios by varying two major source parameters i.e. the minimum halo mass h, min and X-ray photon production efficiency X using the 1D radiative transfer code. We study the impact of variation in these source parameters on the large scale (1 = 0.16 Mpc −1) 21-cm bispectrum for all possible unique triangles in the Fourier domain. Our detailed and comparative analysis of the power spectrum and bispectrum shows that the shape, sign and magnitude of the bispectrum jointly provide a better measure of the signal fluctuations and its non-Gaussianity than the power spectrum alone. We also conclude that it is important to study the sequence of sign changes along with the variations in the shape and magnitude of the bispectrum throughout the CD history to arrive at a robust conclusion about the dominant processes in the intergalactic medium at different cosmic times. We further observe that among all the possible uniquetriangles, the large-scale non-Gaussianity of the signal is best probed by the small-triangles in the squeezed limit and by triangle shapes in its vicinity. This opens up the possibility of constraining the source parameters during the CD using the 21-cm bispectrum.
Journal of Cosmology and Astroparticle Physics, Nov 1, 2022
The emissions from the first luminous sources drive the fluctuations in the 21cm signal at Cosmic... more The emissions from the first luminous sources drive the fluctuations in the 21cm signal at Cosmic Dawn (CD) via two main astrophysical processes, namely Ly coupling and X-ray heating, yielding a highly non-Gaussian signal. The impact of these processes on the 21-cm signal and its non-Gaussianity depends on the properties of these first sources of light. In this work, we consider different CD scenarios by varying two major source parameters i.e. the minimum halo mass h, min and X-ray photon production efficiency X using the 1D radiative transfer code. We study the impact of variation in these source parameters on the large scale (1 = 0.16 Mpc −1) 21-cm bispectrum for all possible unique triangles in the Fourier domain. Our detailed and comparative analysis of the power spectrum and bispectrum shows that the shape, sign and magnitude of the bispectrum jointly provide a better measure of the signal fluctuations and its non-Gaussianity than the power spectrum alone. We also conclude that it is important to study the sequence of sign changes along with the variations in the shape and magnitude of the bispectrum throughout the CD history to arrive at a robust conclusion about the dominant processes in the intergalactic medium at different cosmic times. We further observe that among all the possible uniquetriangles, the large-scale non-Gaussianity of the signal is best probed by the small-triangles in the squeezed limit and by triangle shapes in its vicinity. This opens up the possibility of constraining the source parameters during the CD using the 21-cm bispectrum.
Monthly Notices of the Royal Astronomical Society, May 4, 2019
The upcoming radio interferometer Square Kilometre Array (SKA) is expected to directly detect the... more The upcoming radio interferometer Square Kilometre Array (SKA) is expected to directly detect the redshifted 21-cm signal from the neutral hydrogen present during the Cosmic Dawn. Temperature fluctuations from X-ray heating of the neutral intergalactic medium can dominate the fluctuations in the 21-cm signal from this time. This heating depends on the abundance, clustering, and properties of the X-ray sources present, which remain highly uncertain. We present a suite of three new large-volume, 349 Mpc a side, fully numerical radiative transfer simulations including QSO-like sources, extending the work previously presented in Ross et al. (2017). The results show that our QSOs have a modest contribution to the heating budget, yet significantly impact the 21-cm signal. Initially, the power spectrum is boosted on large scales by heating from the biased QSO-like sources, before decreasing on all scales. Fluctuations from images of the 21-cm signal with resolutions corresponding to SKA1-Low at the appropriate redshifts are well above the expected noise for deep integrations, indicating that imaging could be feasible for all the X-ray source models considered. The most notable contribution of the QSOs is a dramatic increase in non-Gaussianity of the signal, as measured by the skewness and kurtosis of the 21-cm probability distribution functions. However, in the case of late Lyman-α saturation, this non-Gaussianity could be dramatically decreased particularly when heating occurs earlier. We conclude that increased non-Gaussianity is a promising signature of rare X-ray sources at this time, provided that Lyman-α saturation occurs before heating dominates the 21-cm signal.
A number of large current experiments aim to detect the signatures of the Cosmic Reionization at ... more A number of large current experiments aim to detect the signatures of the Cosmic Reionization at redshifts z > 6. Their success depends crucially on understanding the character of the reionization process and its observable consequences and designing the best strategies to use. We use large-scale simulations of cosmic reionization to evaluate the reionization signatures at redshifted 21-cm and small-scale CMB anisotropies in the best current model for the background universe, with fundamental cosmological parameters given by WMAP 3year results (WMAP3). We find that the optimal frequency range for observing the "global step" of the 21-cm emission is 120-150 MHz, while statistical studies should aim at 140-160 MHz, observable by GMRT. Some strongly-nongaussian brightness features should be detectable at frequencies up to ∼ 190 MHz. In terms of sensitivity-signal trade-off relatively low resolutions, corresponding to beams of at least a few arcminutes, are preferable. The CMB anisotropy signal from the kinetic Sunyaev-Zel'dovich effect from reionized patches peaks at tens of µK at arcminute scales and has an rms of ∼ 1µK, and should be observable by the Atacama Cosmology Telescope and the South Pole Telescope. We discuss the various observational issues and the uncertainties involved, mostly related to the poorly-known reionization parameters and, to a lesser extend, to the uncertainties in the background cosmology.
A novel method to characterize the topology of the early-universe intergalactic medium during the... more A novel method to characterize the topology of the early-universe intergalactic medium during the epoch of cosmic reionization is presented. The 21-cm radiation background from high redshift is analyzed through the calculation of the 2-dimensional (2D) genus. The radiative transfer of hydrogen-ionizing photons and ionization-rate equations are calculated in a suite of numerical simulations under various input parameters. The 2D genus is calculated from the mock 21-cm images of the high-redshift Universe. We construct the 2D genus curve by varying the threshold differential brightness temperature, and compare this to the 2D genus curve of the underlying density field. We find that (1) the 2D genus curve reflects the evolutionary track of cosmic reionization and (2) the 2D genus curve can discriminate between certain reionization scenarios and thus indirectly probe the properties of radiation-sources. Choosing the right beam shape of a radio antenna is crucial for this analysis. To this end, the Square Kilometre Array (SKA) is found to be a suitable apparatus for this analysis in terms of sensitivity, even though some deterioration of the data for this purpose is unavoidable under the planned size of the antenna core.
Monthly Notices of the Royal Astronomical Society, Apr 3, 2013
We present detailed radiative transfer simulations of the reionization of the Milky Way by metal-... more We present detailed radiative transfer simulations of the reionization of the Milky Way by metal-poor globular clusters. We identify potential metal-poor globular cluster candidates within the Aquarius simulation using dark matter halo velocity dispersions. We calculate the local ionization fields via a photon-conserving, three dimensional non-equilibrium chemistry code. The key feature of the model is that globular cluster formation is suppressed if the local gas is ionized. We assume that at these early times, the ionization field is dominated by the flux from metal-poor globular clusters. Our spatial treatment of the ionization field leads to drastically different numbers and spatial distributions when compared to models where globular cluster formation is simply truncated at early redshifts (z ∼ 13). The spatial distributions are more extended and more globular clusters are produced. We find that additional sources of ionization are required at later epochs (z ∼ 10) to ionize the remaining gas and recover radial distributions statistically consistent with that of the Milky Way metal-poor globular clusters. We investigate a range of plausible ionization efficiencies to determine the effect photon-rich and photon-poor models have on present-day globular cluster properties. If globular clusters do indeed form within high-redshift dark matter haloes, they produce enough photons to ionize 98 and 90 per cent local (i.e. 2 3 h −3 Mpc 3 centred on the host galaxy) volume and mass by redshift 10, respectively. In our photon-poorest model, this contribution drops to 60 and 50 per cent. Our model therefore implies that globular clusters are important contributors to the reionization process on local scales at high-redshift until more photon-rich sources dominate the photon budget at later times. The surviving clusters in all models have a narrow average age range (mean = 13.34 Gyr, σ = 0.04 Gyr) consistent with current age estimates of the Milky Way metal-poor globular clusters. We also test a simple dynamical destruction model and estimate that ∼60 per cent of all metal-poor globular clusters formed at high redshift have since been destroyed via tidal interactions with the host galaxy.
Monthly Notices of the Royal Astronomical Society, Sep 17, 2007
The reionization of the Universe has profound effects on the way galaxies form and on their obser... more The reionization of the Universe has profound effects on the way galaxies form and on their observed properties at later times. Of particular importance is the relative timing of the reionization history of a region and its halo assembly history, which can affect the nature of the first stars formed in that region, the properties and radial distribution of its stellar halo, globular cluster population and its satellite galaxies. We distinguish two basic cases for the reionization of a halo-internal reionization, whereby the stars forming in situ reionize their host galaxy, and external reionization, whereby the progenitor of a galaxy is reionized by external radiation before its own stars are able to form in sufficient numbers. We use a set of large-scale radiative transfer and structure formation simulations, based on cosmologies derived from both Wilkinson Microwave Anisotropy Probe (WMAP) one-year and WMAP three-year data, to evaluate the mean reionization redshifts and the probability of internal/external reionization for Local Group-like systems, galaxies in the field and central cD galaxies in clusters. We find that these probabilities are strongly dependent on the underlying cosmology and the efficiency of photon production, but also on the halo mass. There is a rapid transition between predominantly external and predominantly internal reionization at a mass scale of ∼10 12 M (corresponding roughly to L * galaxies), with haloes less massive than this being reionized preferentially from distant sources. We provide a fit for the reionization redshift as a function of halo mass, which could be helpful to parametrize reionization in semi-analytical models of galaxy formation on cosmological scales. We find no statistical correlation between the reionization history of field galaxies and their environment.
The prospect of detecting the first galaxies by observing their impact on the intergalactic mediu... more The prospect of detecting the first galaxies by observing their impact on the intergalactic medium (IGM) as they reionized it during the first billion years leads us to ask whether such indirect observations are capable of diagnosing which types of galaxies were most responsible for reionization. We attempt to answer this by extending the galaxy mass range of our first generation of large-scale radiative transfer simulations of reionization in volumes large enough, (100/hM pc) 3 , to make statistically meaningful predictions of observable signatures, downward from those above 10 9 M ⊙ (high-mass, atomic-cooling halos, or "HMACHs") to include those between 10 8 and 10 9 M ⊙ (low-mass, atomic-cooling halos, or "LMACHs"), as well. Previously, we simulated the effects of both HMACHs and LMACHs but only by reducing the box size to 35/h Mpc, too small to apply those simulations to predict observables like the 21-cm background fluctuations. Those simulations showed that LMACHS can make a difference, however. While LMACHs are even more abundant, photoheating suppresses their ability to form stars if they are located inside ionized regions of the IGM, so their contribution starts reionization earlier but tends to saturate over time before it ends. To predict the observables in that case while explicitly tracking the radiation from this full mass range of galaxies, we have had to advance both our N-body and radiative transfer methods substantially, as described here. With this, we perform new simulations in a box 163 Mpc on a side, and focus here on predictions of the 21cm background, to see if upcoming observations are capable of distinguishing a universe ionized primarily by HMACHS from one in which both HMACHs and LMACHs are responsible, and to see how these results depend upon the uncertain source efficiencies. We find that 21-cm fluctuation power spectra observed by the first generation EoR/21cm radio interferometer arrays should be able to distinguish the case of reionization by HMACHs alone from that by both HMACHs and LMACHs, together. Some reionization scenarios, e.g. one with abundant low-efficiency sources vs. one with self-regulation, yield very similar power spectra and rms evolution and thus can only be discriminated by their different mean reionization history and 21-cm PDF distributions. We find that the skewness of the 21-cm PDF distribution smoothed over LOFAR-like window shows a clear feature correlated with the rise of the rms due to patchiness. This is independent on the reionization scenario and thus provides a new approach for detecting the rise of large-scale patchiness and an independent check on other measurements, regardless of the detailed properties of the sources. The peak epoch of the 21-cm rms fluctuations depends significantly on the beam and bandwidth smoothing size as well as on the reionization scenario and can occur for ionized fractions as low as 30% and as high as 70%. Measurements of the mean photoionization rates are sensitive to the average density of the regions being studied and therefore could be strongly skewed in certain cases. Finally, the simulation volume employed has very modest effects on the results during the early and intermediate stages of reionization, but late-time signatures could be significantly affected.
Proceedings of Advancing Astrophysics with the Square Kilometre Array — PoS(AASKA14), 2015
The local intensity of the 21 cm signal emitted during the Epoch of Reionization that will be map... more The local intensity of the 21 cm signal emitted during the Epoch of Reionization that will be mapped by the SKA is modulated by the amount of neutral hydrogen. Consequently, understanding the process of reionization of the intergalactic medium (IGM) is crucial for predicting and interpreting the upcoming observations. After presenting the basic physics and most meaningful quantities pertaining to the process of reionization, we will review recent progress in our understanding of the production and escape of ionizing photons in primordial galaxies and of their absorption in the IGM especially in so-called minihalos and Lyman Limit Systems.
Monthly Notices of the Royal Astronomical Society, 2020
We derive constraints on the thermal and ionization states of the intergalactic medium (IGM) at r... more We derive constraints on the thermal and ionization states of the intergalactic medium (IGM) at redshift ≈ 9.1 using new upper limits on the 21-cm power spectrum measured by the LOFAR radio telescope and a prior on the ionized fraction at that redshift estimated from recent cosmic microwave background (CMB) observations. We have used results from the reionization simulation code grizzly and a Bayesian inference framework to constrain the parameters which describe the physical state of the IGM. We find that, if the gas heating remains negligible, an IGM with ionized fraction ≳0.13 and a distribution of the ionized regions with a characteristic size ≳ 8 h−1 comoving megaparsec (Mpc) and a full width at half-maximum (FWHM) ≳16 h−1 Mpc is ruled out. For an IGM with a uniform spin temperature TS ≳ 3 K, no constraints on the ionized component can be computed. If the large-scale fluctuations of the signal are driven by spin temperature fluctuations, an IGM with a volume fraction ≲0.34 of h...
Monthly Notices of the Royal Astronomical Society, 2019
The upcoming radio interferometer Square Kilometre Array (SKA) is expected to directly detect the... more The upcoming radio interferometer Square Kilometre Array (SKA) is expected to directly detect the redshifted 21-cm signal from the neutral hydrogen present during the Cosmic Dawn. Temperature fluctuations from X-ray heating of the neutral intergalactic medium can dominate the fluctuations in the 21-cm signal from this time. This heating depends on the abundance, clustering, and properties of the X-ray sources present, which remain highly uncertain. We present a suite of three new large-volume, 349 Mpc a side, fully numerical radiative transfer simulations including QSO-like sources, extending the work previously presented in Ross et al. (2017). The results show that our QSOs have a modest contribution to the heating budget, yet significantly impact the 21-cm signal. Initially, the power spectrum is boosted on large scales by heating from the biased QSO-like sources, before decreasing on all scales. Fluctuations from images of the 21-cm signal with resolutions corresponding to SKA1-Low at the appropriate redshifts are well above the expected noise for deep integrations, indicating that imaging could be feasible for all the X-ray source models considered. The most notable contribution of the QSOs is a dramatic increase in non-Gaussianity of the signal, as measured by the skewness and kurtosis of the 21-cm probability distribution functions. However, in the case of late Lyman-α saturation, this non-Gaussianity could be dramatically decreased particularly when heating occurs earlier. We conclude that increased non-Gaussianity is a promising signature of rare X-ray sources at this time, provided that Lyman-α saturation occurs before heating dominates the 21-cm signal.
Monthly Notices of the Royal Astronomical Society, 2014
We study the prospects for constraining the ionized fraction of the intergalactic medium (IGM) at... more We study the prospects for constraining the ionized fraction of the intergalactic medium (IGM) at z > 6 with the next generation of large Lyα emitter surveys. We make predictions for the upcoming Subaru Hyper Suprime-Cam (HSC) Lyα survey and a hypothetical spectroscopic survey performed with the James Webb Space Telescope (JWST). Considering various scenarios where the observed evolution of the Lyα luminosity function of Lyα 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 ∼20 per cent at z = 6.5. If measurements of the Lyα 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 ∼ 7, which would be a strong indication of a partially neutral IGM.
Monthly Notices of the Royal Astronomical Society, 2015
We present estimates of the non-linear bias of cosmological halo formation, spanning a wide range... more We present estimates of the non-linear bias of cosmological halo formation, spanning a wide range in the halo mass from ∼10 5 to ∼10 12 M , based upon both a suite of high-resolution cosmological N-body simulations and theoretical predictions. The halo bias is expressed in terms of the mean bias and stochasticity as a function of local overdensity (δ), under different filtering scales, which is realized as the density of individual cells in uniform grids. The sampled overdensities span a range wide enough to provide the fully non-linear bias effect on the formation of haloes. A strong correlation between δ and halo population overdensity δ h is found, along with sizable stochasticity. We find that the empirical mean halo bias matches, with good accuracy, the prediction by the peak-background split method based on the excursion set formalism, as long as the empirical, globally averaged halo mass function is used. Consequently, this bias formalism is insensitive to uncertainties caused by varying halo-identification schemes, and can be applied generically. We also find that the probability distribution function of biased halo numbers has wider distribution than the pure Poisson shot noise, which is attributed to the sub-cell-scale halo correlation. We explicitly calculate this correlation function and show that both overdense and underdense regions have positive correlation, leading to stochasticity larger than the Poisson shot noise in the range of haloes and halo-collapse epochs we study.
In recent years there has been considerable progress in our understanding of the nature and prope... more In recent years there has been considerable progress in our understanding of the nature and properties of the reionization process. In particular, the numerical simulations of this epoch have made a qualitative leap forward, reaching sufficiently large scales to derive the characteristic scales of the reionization process and thus allowing for realistic observational predictions. Our group has recently performed the first such large-scale radiative transfer simulations of reionization, run on top of state-of-the-art simulations of early structure formation. This allowed us to make the first realistic observational predictions about the Epoch of Reionization based on detailed radiative transfer and structure formation simulations. We discuss the basic features of reionization derived from our simulations and some recent results on the observational implications for the high-redshift Ly-α sources.
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Papers by Ilian T Iliev