Ingredients for 21 cm Intensity Mapping

Abstract

Current and upcoming radio telescopes will map the spatial distribution of cosmic neutral hydrogen (H I) through its 21 cm emission. In order to extract the maximum information from these surveys, accurate theoretical predictions are needed. We study the abundance and clustering properties of H I at redshifts z5 using TNG100, a large state-of-the-art magnetohydrodynamic simulation of a 75h −1 Mpc box size, which is part of the IllustrisTNG Project. We show that most of the H I lies within dark matter halos, and we provide fits for the halo H I mass function, i.e.,the mean H I mass hosted by a halo of mass M at redshift z. We find that only halos with circular velocities larger than ;30km s −1 contain H I. While the density profiles of H I exhibit a large halo-to-halo scatter, the mean profiles are universal across mass and redshift. The H I in low-mass halos is mostly located in the central galaxy, while in massive halos the H I is concentrated in the satellites. Our simulation reproduces the bias value of damped Lyα systems from observations. We show that the H I and matter density probability distribution functions differ significantly. Our results point out that for small halos, the H I bulk velocity goes in the same direction and has the same magnitude as the halo peculiar velocity, while in large halos, differences show up. We find that halo H I velocity dispersion follows a power law with halo mass. We find a complicated H I bias, with H I already becoming nonlinear at k=0.3 h Mpc −1 at z3. The clustering of H I can, however, be accurately reproduced by perturbative methods. We find a new secondary bias by showing that the clustering of halos depends not only on mass but also on H I content. We compute the amplitude of the H I shot noise and find that it is small at all redshifts, verifying the robustness of BAO measurements with 21 cm intensity mapping. We study the clustering of H I in redshift space and show that linear theory can explain the ratio between the monopoles in redshift and real space down to 0.3, 0.5, and 1 h Mpc −1 at redshifts 3, 4, and 5, respectively. We find that the amplitude of the Fingers-of-God effect is larger for H I than for matter, since H I is found only in halos above a certain mass. We point out that 21 cm maps can be created from N-body simulations rather than full hydrodynamic simulations. Modeling the one-halo term is crucial for achieving percent accuracy with respect to a full hydrodynamic treatment. Although our results are not converged against resolution, they are, however, very useful as we work at the resolution where the model parameters have been calibrated to reproduce galaxy properties.

The atomic hydrogen content of the post-reionization Universe

“…In contrast, hydro-dynamical simulations generally predict an opposite trend or significantly weaker evolution (e.g. Davé et al 2017;Villaescusa-Navarro et al 2018). At high redshift, the observational measurements are based on Damped Lyman-α systems (DLAs) identified in the spectra of bright quasars.…”

“…A detailed characterization of the HI content of dark matter haloes is a fundamental ingredient to make predictions for the 21 cm signal. This is particularly relevant for intensity mapping, especially in the framework of the halo model (Castorina & Villaescusa-Navarro 2017;Villaescusa-Navarro et al 2018). Hydro-dynamical simulations and SAMs are not only a valuable tool to model the relation between HI and halo mass, but also offer the possibility of investigating the physical origin of this relation.…”

“…In these methods, the HI content of dark matter haloes depends only on halo mass -this defines the HI halo mass function M HI (M h ) -and all the quantities described above can be related to this quantity (e.g. Santos et al 2015;Villaescusa-Navarro et al 2018). Halo catalogues obtained with approximate methods such as Lagrangian Perturbation Theory (e.g Monaco et al 2002) can be used to populate the dark matter haloes with HI, in combination with parametrizations for the HI halo mass function.…”

“…An alternative approach to model the HI distribution in a cosmological context is based on hydro-dynamical simulations (e.g. Duffy et al 2012;Zavala et al 2016;Davé et al 2013;Villaescusa-Navarro et al 2018). Physical processes like star formation, feedback from stellar winds, supernovae and Active Galactic Nuclei (AGN), black hole accretion, etc.…”

Anisotropic halo assembly bias and redshift-space distortions

“…In order to investigate in detail the internal properties of halos and their correlations with the large-scale tidal field, we use 1000 N-body simulations, a subset of Quijote 1 suite [30]. The subset we use are the output snapshots at z = 0 from the dark matter only simulations run using TreePM+SPH code Gadget-III [31] in a periodic box size of 1 h −1 Gpc with 512 3 particles.…”