The Atlas3D project -- IV: the molecular gas content of early-type
  galaxies
  We have carried out a survey for 12CO J=1-0 and J=2-1 emission in the 260
early-type galaxies of the volume-limited Atlas3D sample, with the goal of
connecting their star formation and assembly histories to their cold gas
content. This is the largest volume-limited CO survey of its kind and is the
first to include many Virgo Cluster members. Sample members are dynamically hot
galaxies with a median stellar mass 3\times 10^{10} Msun; they are selected by
morphology rather than colour, and the bulk of them lie on the red sequence.
The overall CO detection rate is 56/259 = 0.22 \error 0.03, with no dependence
on K luminosity and only a modest dependence on dynamical mass. There are a
dozen CO detections among the Virgo Cluster members; statistical analysis of
their H_2 mass distributions and their dynamical status within the cluster
shows that the cluster's influence on their molecular masses is subtle at best,
even though (unlike spirals) they seem to be virialized within the cluster. We
suggest that the cluster members have retained their molecular gas through
several Gyr residences in the cluster. There are also a few extremely CO-rich
early-type galaxies with H_2 masses >= 10^9 Msun, and these are in low density
environments. We do find a significant trend between molecular content and the
stellar specific angular momentum. The galaxies of low angular momentum also
have low CO detection rates, suggesting that their formation processes were
more effective at destroying molecular gas or preventing its re-accretion. We
speculate on the implications of these data for the formation of various
sub-classes of early-type galaxies.
