The cosmic atomic hydrogen mass density as a function of mass and galaxy
  hierarchy from spectral stacking
  We use spectral stacking to measure the contribution of galaxies of different
masses and in different hierarchies to the cosmic atomic hydrogen (HI) mass
density in the local Universe. Our sample includes 1793 galaxies at $z < 0.11$
observed with the Westerbork Synthesis Radio Telescope, for which Sloan Digital
Sky Survey spectroscopy and hierarchy information are also available. We find a
cosmic HI mass density of $\Omega_{\rm HI} = (3.99 \pm 0.54)\times 10^{-4}
h_{70}^{-1}$ at $\langle z\rangle = 0.065$. For the central and satellite
galaxies, we obtain $\Omega_{\rm HI}$ of $(3.51 \pm 0.49)\times 10^{-4}
h_{70}^{-1}$ and $(0.90 \pm 0.16)\times 10^{-4} h_{70}^{-1}$, respectively. We
show that galaxies above and below stellar masses of $\sim$10$^{9.3}$
M$_{\odot}$ contribute in roughly equal measure to the global value of
$\Omega_{\rm HI}$. While consistent with estimates based on targeted HI
surveys, our results are in tension with previous theoretical work. We show
that these differences are, at least partly, due to the empirical recipe used
to set the partition between atomic and molecular hydrogen in semi-analytical
models. Moreover, comparing our measurements with the cosmological
semi-analytic models of galaxy formation {\sc Shark} and GALFORM reveals
gradual stripping of gas via ram pressure works better to fully reproduce the
properties of satellite galaxies in our sample, than strangulation. Our
findings highlight the power of this approach in constraining theoretical
models, and confirm the non-negligible contribution of massive galaxies to the
HI mass budget of the local Universe.
