Extreme spheres: countsincells for 21cm intensity mapping
Abstract
Intensity mapping surveys will provide access to a coarse view of the cosmic largescale structure at high redshifts. Given the large fraction of the sky that can be efficiently scanned using emission from cosmic neutral hydrogen (H I), intensity mapping is ideally suited to probe a wide range of density environments and hence to constrain cosmology and fundamental physics. To efficiently extract information from 21 cm intensities beyond average, one needs nonGaussian statistics that capture large deviations from the mean H I density. Countsincells are ideally suited for this purpose, as their statistics can be predicted accurately. We use a large stateoftheart magnetohydrodynamic simulation from the IllustrisTNG project to determine the relation between neutral hydrogen and matter densities in cells. We demonstrate how our theoretical knowledge about the matter probability density function (PDF) for a given cosmology can be used to extract a parametrizationindependent H I bias function from a measured H I PDF. Inspired by the shape of this bias function, we employ a simple quadratic approximation that reproduces the measured bias function at a few per cent level. Combining this quadratic bias function with the predicted matter PDF yields a fully predictive forward model for the H I PDF that matches the measured H I PDF at a few per cent accuracy at scale R = 5 Mpc h^{1} from redshift z = 5 to z = 1. Furthermore, we find a densitydependent H I clustering signal that is consistent with theoretical expectations and could allow for joint constraints of H I bias and the amplitude of matter fluctuations or the growth of structure.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 March 2019
 DOI:
 10.1093/mnras/sty3469
 arXiv:
 arXiv:1808.09968
 Bibcode:
 2019MNRAS.484..269L
 Keywords:

 methods: analytical;
 methods: numerical;
 largescale structure of Universe;
 cosmology: theory;
 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 doi:10.1093/mnras/sty3469