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Register a new userModelling the post-reionization neutral Hydrogen (HI) bias
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Debanjan Sarkar
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Observations of the neutral Hydrogen (HI ) 21-cm signal hold the potential of allowing us to map out the cosmological large scale structures (LSS) across the entire post-reionization era ($z leq 6$). Several experiments are planned to map the LSS over a large range of redshifts and angular scales, many of these targeting the Baryon Acoustic Oscillations. It is important to model the HI distribution in order to correctly predict the expected signal, and more so to correctly interpret the results after the signal is detected. In this paper we have carried out semi-numerical simulations to model the HI distribution and study the HI power spectrum $P_{HI}(k,z)$ across the redshift range $1 le z le 6$. We have modelled the HI bias as a complex quantity $tilde{b}(k,z)$ whose modulus squared $b^2(k,z)$ relates $P_{HI}(k,z)$ to the matter power spectrum $P(k,z)$, and whose real part $b_r(k,z)$ quantifies the cross-correlation between the HI and the matter distribution. We study the $z$ and $k$ dependence of the bias, and present polynomial fits which can be used to predict the bias across $0 le z le6$ and $0.01 le k le 10 , {rm Mpc}^{-1}$. We also present results for the stochasticity $r=b_r/b$ which is important for cross-correlation studies.
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Measurements of the post-reionization 21-cm bispectrum $B_{{rm HI}}(mathbf{k_1},mathbf{k_2},mathbf{k_3})$ using various upcoming intensity mapping experiments hold the potential for determining the cosmological parameters at a high level of precision. In this paper we have estimated the 21-cm bispectrum in the $z$ range $1 le z le 6$ using semi-numerical simulations of the neutral hydrogen (${rm HI}$) distribution. We determine the $k$ and $z$ range where the 21-cm bispectrum can be adequately modelled using the predictions of second order perturbation theory, and we use this to predict the redshift evolution of the linear and quadratic ${rm HI}$ bias parameters $b_1$ and $b_2$ respectively. The $b_1$ values are found to decreases nearly linearly with decreasing $z$, and are in good agreement with earlier predictions obtained by modelling the 21-cm power spectrum $P_{{rm HI}}(k)$. The $b_2$ values fall sharply with decreasing $z$, becomes zero at $z sim 3$ and attains a nearly constant value $b_2 approx - 0.36$ at $z<2$. We provide polynomial fitting formulas for $b_1$ and $b_2$ as functions of $z$. The modelling presented here is expected to be useful in future efforts to determine cosmological parameters and constrain primordial non-Gaussianity using the 21-cm bispectrum.
The post-reionization ${rm HI}$ 21-cm signal is an excellent candidate for precision cosmology, this however requires accurate modelling of the expected signal. Sarkar et al. (2016) have simulated the real space ${rm HI}$ 21-cm signal, and have modelled the ${rm HI}$ power spectrum as $P_{{rm HI}}(k)=b^2 P(k)$ where $P(k)$ is the dark matter power spectrum and $b(k)$ is a (possibly complex) scale dependent bias for which fitting formulas have been provided. This paper extends these simulations to incorporate redshift space distortion and predict the expected redshift space ${rm HI}$ 21-cm power spectrum $P^s_{{rm HI}}(k_{perp},k_{parallel})$ using two different prescriptions for the ${rm HI}$ distributions and peculiar velocities. We model $P^s_{{rm HI}}(k_{perp},k_{parallel})$ assuming that it is the product of $P_{{rm HI}}(k)=b^2 P(k)$ with a Kaiser enhancement term and a Finger of God (FoG) damping which has $sigma_p$ the pair velocity dispersion as a free parameter. Considering several possibilities for the bias and the damping profile, we find that the models with a scale dependent bias and a Lorentzian damping profile best fit the simulated $P^s_{{rm HI}}(k_{perp},k_{parallel})$ over the entire range $1 le z le 6$. The best fit value of $sigma_p$ falls approximately as $(1+z)^{-m}$ with $m=2$ and $1.2$ respectively for the two different prescriptions. The model predictions are consistent with the simulations for $k < 0.3 , {rm Mpc}^{-1}$ over the entire $z$ range for the monopole $P^s_0(k)$, and at $z le 3$ for the quadrupole $P^s_2(k)$. At $z ge 4$ the models underpredict $P^s_2(k)$ at large $k$, and the fit is restricted to $k < 0.15 , {rm Mpc}^{-1}$.
The neutral hydrogen (HI) and its 21 cm line are promising probes to the reionization process of the intergalactic medium (IGM). To use this probe effectively, it is imperative to have a good understanding on how the neutral hydrogen traces the underlying matter distribution. Here we study this problem using semi-numerical modeling by combining the HI in the IGM and the HI from halos during the epoch of reionization (EoR), and investigate the evolution and the scale-dependence of the neutral fraction bias as well as the 21 cm line bias. We find that the neutral fraction bias on large scales is negative during reionization, and its absolute value on large scales increases during the early stage of reionization and then decreases during the late stage. During the late stage of reionization, there is a transition scale at which the HI bias transits from negative on large scales to positive on small scales, and this scale increases as the reionization proceeds to the end.
The spatial distribution of neutral hydrogen (HI) in the Universe contains a wealth of cosmological information. The 21 cm emission line can be used to map the HI up to very high redshift and therefore reveal us something about the evolution of the large scale structures in the Universe. However little is known about the abundance and clustering properties of the HI over cosmic time. Motivated by this, we build an analytic framework where the relevant parameters that govern how the HI is distributed among dark matter halos can be fixed using observations. At the same time we provide tools to study the column density distribution function of the HI absorbers together with their clustering properties. Our formalism is the first one able to account for all observations at a single redshift, $z = 2.3$. The linear bias of the HI and the mean number density of HI sources, two main ingredients in the calculation of the signal-to-noise ratio of a cosmological survey, are then discussed in detail, also extrapolating the results to low and high redshift. We find that HI bias is relatively higher than the value reported in similar studies, but the shot noise level is always sub dominant, making the HI Power Spectrum always a high signal-to-noise measurements up to $zsimeq5$ in the limit of no instrumental noise and foreground contamination.
The epoch of reionization is one of the least known chapters in the evolutionary history of the Universe. This thesis investigates two major approaches to unveil the reionization history of the Universe using HI 21-cm maps.The most discussed approach has been to study the global statistical properties of the reionization HI 21-cm. We develop the formalism to calculate the Multi-frequency Angular Power Spectrum (MAPS) and quantify the statistics of the HI signal as a joint function of the angular multipole l and frequency separation Delta u. We adopt a simple model for the HI distribution which incorporates patchy reionization and use it to study the signatures of ionized bubbles on MAPS. We also study the implications of the foreground subtraction. A major part of the thesis investigates the possibility of detecting ionized bubbles around individual sources in 21-cm maps. We present a visibility based matched filter technique to optimally combine the signal from an ionized bubble and minimize the noise and foreground contributions. The formalism makes definite predictions on the ability to detect an ionized bubble or conclusively rule out its presence within a radio map. Results are presented for the GMRT and the MWA. Using simulated HI maps we analyzed the impact of HI fluctuations outside the bubble on its detectability. Various other issues such as (i) bubble size determination (ii) blind search for bubbles, (iii) optimum redshift for bubble detection are also discussed.
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