The Atlas3D project -- XXVII. Cold Gas and the Colours and Ages of
  Early-type Galaxies
  We present a study of the cold gas contents of the Atlas3D early-type
galaxies, in the context of their optical colours, near-UV colours, and H\beta\
absorption line strengths. Early-type (elliptical and lenticular) galaxies are
not as gas-poor as previously thought, and at least 40% of local early-type
galaxies are now known to contain molecular and/or atomic gas. This cold gas
offers the opportunity to study recent galaxy evolution through the processes
of cold gas acquisition, consumption (star formation), and removal. Molecular
and atomic gas detection rates range from 10% to 34% in red sequence early-type
galaxies, depending on how the red sequence is defined, and from 50% to 70% in
blue early-type galaxies. Notably, massive red sequence early-type galaxies
(stellar masses > 5e10 Msun, derived from dynamical models) are found to have
HI masses up to M(HI)/Mstar ~ 0.06 and H_2 masses up to M(H$_2$)/Mstar ~ 0.01.
Some 20% of all massive early-type galaxies may have retained atomic and/or
molecular gas through their transition to the red sequence. However, kinematic
and metallicity signatures of external gas accretion (either from satellite
galaxies or the intergalactic medium) are also common, particularly at stellar
masses <= 5e10 Msun, where such signatures are found in ~ 50% of H$_2$-rich
early-type galaxies. Our data are thus consistent with a scenario in which fast
rotator early-type galaxies are quenched former spiral galaxies which have
undergone some bulge growth processes, and in addition, some of them also
experience cold gas accretion which can initiate a period of modest star
formation activity. We discuss implications for the interpretation of
colour-magnitude diagrams.
