The 21-cm signal during the end stages of reionization

During the epoch of reionization (EoR), the 21-cm signal allows direct observation of the neutral hydrogen (H I) in the intergalactic medium (IGM). In the post-reionization era, this signal inst≲ead probes H I in galaxies, which traces the dark matter density distribution. With new numerical simulations, we investigated the end stages of reionization to elucidate the transition of our Universe into the post-reionization era. Our models are consistent with the latest high-redshift measurements, including ultraviolet (UV) luminosity functions up to redshift ≃8. Notably, these models consistently reproduced the evolution of the UV photon background, which is constrained from Lyman-α absorption spectra. We studied the dependence of this background on the nature of photon sinks in the IGM, requiring mean free path of UV photons to be ∼10 comoving-megaparsecs (cMpc) during the EoR that increases gradually with time during late stages (z ≲ 6). Our models revealed that the reionization of the IGM transitioned from an inside-out to an outside-in process when the Universe is less than 0.01 per cent neutral. During this epoch, the 21-cm signal also shifted from probing predominantly the H I in the IGM to that in galaxies. Furthermore, we identified a statistically significant number of large neutral islands (with sizes up to 40 cMpc) persisting until very late stages (5 z ≲ 6) that can imprint features in Lyman-α absorption spectra and also produce a knee-like feature in the 21-cm power spectrum.</p

The wedge bias in reionization 21-cm power spectrum measurements

A proposed method for dealing with foreground emission in upcoming 21-cm observations from the epoch of reionization is to limit observations to an uncontaminated window in Fourier space. Foreground emission can be avoided in this way, since it is limited to a wedge-shaped region in k?, k? space. However, the power spectrum is anisotropic owing to redshift-space distortions from peculiar velocities. Consequently, the 21-cm power spectrum measured in the foreground avoidance window – which samples only a limited range of angles close to the line-of-sight direction – differs from the full redshift-space spherically averaged power spectrum which requires an average over all angles. In this paper, we calculate the magnitude of this ‘wedge bias’ for the first time. We find that the bias amplifies the difference between the real-space and redshift-space power spectra. The bias is strongest at high redshifts, where measurements using foreground avoidance will overestimate the redshift-space power spectrum by around 100 per cent, possibly obscuring the distinctive rise and fall signature that is anticipated for the spherically averaged 21-cm power spectrum. In the later stages of reionization, the bias becomes negative, and smaller in magnitude (?20 per cent)

On the use of Lya emitters as probes of reionization

We use numerical simulations to study the effects of the patchiness of a partly reionized intergalactic medium (IGM) on the observability of Lya emitters (LAEs) at high redshifts (z ? 6). We present a new model that divides the Lya 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 Lya 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 ~ 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 ~40 per cent ionized at z = 7, ~50 per cent ionized at z = 6.5 and ~100 per cent ionized at z = 6. This is in conflict with other observations, suggesting that intrinsic LAE evolution at z ? 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

Impact of the Epoch of Reionization sources on the 21-cm bispectrum

The morphology of the 21-cm signal emitted by the neutral hydrogen present in the intergalactic medium (IGM) during the Epoch of Reionization (EoR) depends both on the properties of the sources of ionizing radiation and on the underlying physical processes within the IGM. Variation in the morphology of the IGM 21-cm signal due to the different sources of the EoR is expected to have a significant impact on the 21-cm bispectrum, which is one of the crucial observable statistics that can evaluate the non-Gaussianity present in the signal and which can be estimated from radio interferometric observations of the EoR. Here we present the 21-cm bispectrum for different reionization scenarios assuming different simulated models for the sources of reionization. We also demonstrate how well the 21-cm bispectrum can distinguish between different IGM 21-cm signal morphologies, arising due to the differences in the reionization scenarios, which will help us shed light on the nature of the sources of ionizing photons. Our estimated large-scale bispectrum for all unique k-triangle shapes shows a significant difference in the magnitude and sign across different reionization scenarios. Additionally, our focused analysis of bispectrum for a few specific k-triangle shapes (e.g. squeezed-limit, linear, and shapes in the vicinity of the squeezed-limit) shows that the large scale 21-cm bispectrum can distinguish between reionization scenarios that show inside-out, outside-in and a combination of inside-out and outside-in morphologies. These results highlight the potential of using the 21-cm bispectrum for constraining different reionization scenarios.</p

POLAR - I: linking the 21-cm signal from the epoch of reionization to galaxy formation

To self-consistently model galactic properties, reionization of the intergalactic medium, and the associated 21-cm signal, we have developed the algorithm POLAR by integrating the one-dimensional radiative transfer code GRIZZLY with the semi-analytical galaxy formation code L-GALAXIES 2020. Our proof-of-concept results are consistent with observations of the star formation rate history, UV luminosity function, and the CMB Thomson scattering optical depth. We then investigate how different galaxy formation models affect UV luminosity functions and 21-cm power spectra, and find that while the former are most sensitive to the parameters describing the merger of haloes, the latter have a stronger dependence on the supernovae feedback parameters, and both are affected by the escape fraction model.</p