The origin of the atomic and molecular gas contents of early-type
  galaxies. I. A new test of galaxy formation physics
  We study the atomic (HI) and molecular hydrogen (H2) contents of early-type
galaxies (ETGs) and their gas sources using the GALFORM model of galaxy
formation. This model uses a self-consistent calculation of the star formation
rate (SFR), which depends on the H2 content of galaxies. We first present a new
analysis of HIPASS and ATLAS3D surveys, with special emphasis on ETGs. The
model predicts HI and H2 contents of ETGs in agreement with the observations
from these surveys only if partial ram pressure stripping of the hot gas is
included, showing that observations of neutral gas in `quenched' galaxies place
stringent constraints on the treatment of the hot gas in satellites. We find
that ~90% of ETGs at z=0 have neutral gas contents supplied by radiative
cooling from their hot halos, 8% were supplied by gas accretion from minor
mergers that took place in the last 1Gyr, while 2% were supplied by mass loss
from old stars. The model predicts neutral gas fractions strongly decreasing
with increasing bulge fraction. This is due to the impeded disk regeneration in
ETGs, resulting from both active galactic nuclei feedback and environmental
quenching by partial ram pressure stripping of the hot gas.
