Large-Scale Structure with 21cm Intensity Mapping

We are witnessing exciting times in the field of cosmology. Current and future experiments and surveys will provide us with tight constraints on the key cosmological parameters. A new and promising technique of mapping the Large-Scale Structure (LSS) in our Universe is the 21cm Intensity Mapping (IM) in which one uses the emission of the neutral hydrogen as a tracer of the underlying matter field. In principle this technique can be used to map huge portions of our Universe and deliver 3D structure measurements providing us with the information that is complementary to the information extracted from the Cosmic Microwave Background (CMB) experiments. However, the field of 21cm IM cosmology is still in its raising and is severely limited by the foreground issues and problems. In this thesis we will consider several aspects of using the 21cm IM as an LSS probe in order to better constrain the cosmological parameters. First, we present and analyse a Baryon Acoustic Oscillation (BAO) reconstruction method that consists of displacing pixels instead of galaxies and whose implementation is easier than the standard reconstruction method. We show that this method is equivalent to the standard reconstruction technique in the limit where the number of pixels becomes very large. This method is particularly useful in surveys where individual galaxies are not resolved, as in 21cm IM observations. We validate this method by reconstructing mock pixelated maps, that we build from the distribution of matter and halos in real- and redshift-space, from a large set of numerical simulations. We find that this method is able to decrease the uncertainty in the BAO peak position by 30-50% over the typical angular resolution scales of 21cm IM experiments. Second, we investigate the possibility of performing cosmological studies in the redshift range 2.5 < z < 5 through suitable extensions of existing and upcoming radio-telescopes like CHIME, HIRAX and FAST. We use the Fisher matrix technique to forecast the bounds that those instruments can place on the growth rate, the BAO distance scale parameters, the sum of the neutrino masses and the number of relativistic degrees of freedom at decoupling, Neff. We point out that quantities that depend on the amplitude of the 21cm power spectrum, like fσ8, are completely degenerate with ΩHI and bHI. Then, we propose several strategies to independently constrain them through cross-correlations with other probes. We study in detail the dependence of our results on the instrument, amplitude of the HI bias, the foreground wedge coverage, the nonlinear scale used in the analysis, uncertainties in the theoretical modeling and the priors on bHI and ΩHI. We conclude that 21cm IM surveys operating in this redshift range can provide extremely competitive constraints on key cosmological parameters. Thridly, we have used TNG100, a large state-of-the-art magneto-hydrodynamic simulation of a 75 h−1 Mpc box size, which is part of the IllustrisTNG Project, to study the neutral hydrogen density profiles in dark matter halos. We find that while the density profiles of HI exhibit a large halo-to-halo scatter, the mean profiles are universal across mass and redshift. Finally, we combine information from the clustering of HI galaxies in the 100% data release of the Arecibo Legacy Fast ALFA survey (ALFALFA), and from the HI content of optically-selected galaxy groups found in the Sloan Digital Sky Survey (SDSS) to constrain the relation between halo mass Mh and its average total HI mass content MHI. We model the abundance and clustering of neutral hydrogen through a halo-model-based approach, parametrizing the MHI(Mh) relation as a power law with an exponential mass cutoff. To break the degeneracy between the amplitude and low-mass cutoff of the MHI(Mh) relation, we also include a recent measurement of the cosmic HI abundance from the 100% ALFALFA sample. We find that all datasets are consistent with a power-law index α = 0.44±0.08 and a cutoff halo mass log10 Mmin /(h^−1M⊙) = 11.27+0.24−0.30. We compare these results with predictions from state-of-the-art magneto-hydrodynamical simulations, and find both to be in good qualitative agreement, although the data favours a significantly larger cutoff mass that is consistent with the higher cosmic HI abundance found in simulations. Both data and simulations seem to predict a similar value for the HI bias (bHI = 0.875 ± 0.022) and shot-noise power (PSN = 92+20-18 [h^−1Mpc]^3) at redshift z = 0.