The ATLAS3D project - XXII. Low-efficiency star formation in early-type
  galaxies: hydrodynamic models and observations
  We study the global efficiency of star formation in high resolution
hydrodynamical simulations of gas discs embedded in isolated early-type and
spiral galaxies. Despite using a universal local law to form stars in the
simulations, we find that the early-type galaxies are offset from the spirals
on the large-scale Kennicutt relation, and form stars 2 to 5 times less
efficiently. This offset is in agreement with previous results on morphological
quenching: gas discs are more stable against star formation when embedded in
early-type galaxies due to the lower disc self-gravity and increased shear. As
a result, these gas discs do not fragment into dense clumps and do not reach as
high densities as in the spiral galaxies. Even if some molecular gas is
present, the fraction of very dense gas (above 10^4 cm-3) is significantly
reduced, which explains the overall lower star formation efficiency. We also
analyse a sample of local early-type and spiral galaxies, measuring their CO
and HI surface densities and their star formation rates as determined by their
non-stellar 8um emission. As predicted by the simulations, we find that the
early-type galaxies are offset from the Kennicutt relation compared to the
spirals, with a twice lower efficiency. Finally, we validate our approach by
performing a direct comparison between models and observations. We run a
simulation designed to mimic the stellar and gaseous properties of NGC524, a
lenticular galaxy, and find a gas disc structure and global star formation rate
in good agreement with the observations. Morphological quenching thus seems to
be a robust mechanism, and is also consistent with other observations of a
reduced star formation efficiency in early-type galaxies in the COLD GASS
survey. This lower efficiency of star formation is not enough to explain the
formation of the whole Red Sequence, but can contribute to the reddening of
some galaxies.
