Neutral Hydrogen in the Local Universe

We revisit the composition of neutral hydrogen, i.e. atomic (HI) and molecular $(H2),$ in the local Universe, with focus on the $H2$‐to‐HI mass ratio $ηgal$ in various galaxies and the $H2$‐mass function (MF). First, we derive the $H2$‐MF from the CO‐luminosity function (LF) of the FCRAO Extragalactic CO Survey, using a variable CO‐to‐$H2$ conversion fitted to nearby observations. This implies a universal $H2$ density $ΩH2 = (6.9±2.7)⋅10−5 h−1$ and $ΩH2/ΩHI = 0.26±0.11,$ which could, however, suffer from incompleteness of the CO‐sample. Therefore, a second approach derives the $H2$‐MF from a HI‐sample with well described completeness (HIPASS). This premisses an estimation of $ηgal,$ for which we introduce a set of phenomenological models based on a sample of 245 galaxies drawn from the literature. Our best model in terms of statistical scatter describes $ηgal$ as a function of galaxy type and cold gas mass. This model leads to an $H2$‐MF that matches the one derived from the CO‐LF, thus supporting that model and settling completeness uncertainties.