Speaker
Description
Atomic hydrogen (HI) gas, mostly residing in dark matter halos after cosmic reionization, is the fuel for star formation. Its relation with properties of host halo is the key to understand the cosmic HI distribution. In this work, we propose a flexible, empirical model of HI-halo relation. In this model, while the HI mass depends primarily on the mass of host halo, there is also secondary dependence on other halo properties. We apply our model to the observation data of the Arecibo Fast Legacy ALFA Survey (ALFALFA), and find it can successfully fit to the cosmic HI abundance ($\Omega_{\rm HI}$), average HI-halo mass relation $\langle M_{\rm HI}|M_{\rm h}\rangle$, and the HI clustering. The bestfit of the ALFALFA data rejects with high confidence level the model with no secondary halo dependence of HI mass and the model with secondary dependence on halo spin parameter ($\lambda$), and shows strong dependence on halo formation time ($a_{1/2}$) and halo concentration ($c_{\rm vir}$). In attempt to explain these findings from the perspective of hydrodynamical simulations, the IllustrisTNG simulation confirms the dependence of HI mass on secondary halo parameters. However, the IllustrisTNG results show strong dependence on $\lambda$ and weak dependence on $c_{\rm vir}$ and $a_{1/2}$, and also predict a much larger value of HI clustering on large scales than observations. This discrepancy between the simulation and observation calls for improvements in understanding the HI-halo relation from both theoretical and observational sides.
