Direct Measurement of the HI-Halo Mass Relation through Stacking

We present accurate measurements of the total HI mass in dark matter halos of different masses at z ~ 0, by stacking the HI spectra of entire groups from the Arecibo Fast Legacy ALFA Survey. The halos are selected from the optical galaxy group catalog constructed from the Sloan Digital Sky Survey DR7 Main Galaxy sample, with reliable measurements of halo mass and halo membership. We find that the HI-halo mass relation is not a simple monotonic function, as assumed in several theoretical models. In addition to the dependence of halo mass, the total HI gas mass shows strong dependence on the halo richness, with larger HI masses in groups with more members at fixed halo masses. Moreover, halos with at least three member galaxies in the group catalog have a sharp decrease of the HI mass, potentially caused by the virial halo shock-heating and the AGN feedback. The dominant contribution of the HI gas comes from the central galaxies for halos of $M<10^{12.5}h^{-1}M_{\odot}$, while the satellite galaxies dominate over more massive halos. Our measurements are consistent with a three-phase formation scenario of the HI-rich galaxies. The smooth cold gas accretion is driving the HI mass growth in halos of $M<10^{11.8}h^{-1}M_{\odot}$, with late-forming halos having more HI accreted. The virial halo shock-heating and AGN feedback will take effect to reduce the HI supply in halos of $10^{11.8}h^{-1}M_{\odot}<M<10^{13}h^{-1}M_{\odot}$. The HI mass in halos more massive than $10^{13}h^{-1}M_{\odot}$ generally grows by mergers, with the dependence on halo richness becoming much weaker.