Modeling the neutral hydrogen distribution in the post-reionization Universe: intensity mapping

We model the distribution of neutral hydrogen (HI) in the post-reionization era and investigate its detectability in 21 cm intensity mapping with future radio telescopes like the Square Kilometer array (SKA). We rely on high resolution hydrodynamical N-body simulations that have a state-of-the-art treatment of the low density photoionized gas in the inter-galactic medium (IGM). The HI is assigned a-posteriori to the gas particles following two different approaches: a halo-based method in which HI is assigned only to gas particles residing within dark matter halos; a particle-based method that assigns HI to all gas particles using a prescription based on the physical properties of the particles. The HI statistical properties are then compared to the observational properties of Damped Lyman-alpha Absorbers (DLAs) and of lower column density systems and reasonable good agreement is found for all the cases. Among the halo-based method, we further consider two different schemes that aim at reproducing the observed properties of DLAs by distributing HI inside halos: one of this results in a much higher bias for DLAs, in agreement with recent observations, which boosts the 21 cm power spectrum by a factor similar to 4 with respect to the other recipe. Furthermore, we quantify the contribution of HI in the diffuse IGM to both Omega(HI) and the HI power spectrum finding to be subdominant in both cases. We compute the 21 cm power spectrum from the simulated HI distribution and calculate the expected signal for both SKAl-mid and SKAl-low configurations at 2.4 <= z <= 4. We find that SKA will be able to detect the 21 cm power spectrum, in the non-linear regime, up to k similar to 1h/Mpc for SKAl-mid and k similar to 5h/Mpc for SKAl-low with 100 hours of observations. We also investigate the perspective of imaging the HI distribution. Our findings indicate that SKAl-low could detect the most massive HI peaks with a signal to noise ratio (SNR) higher than 5 for an observation time of about 1000 hours at z = 4, for a synthesized beam width of 2'. Detection at redshifts z >= 2.4 with SKAl-mid would instead require a much longer observation time to achieve a comparable SNR level.