We present a study of the cold gas contents of the Atlas ^ { \mathrm { 3 D } } 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 > 5 \times 10 ^ { 10 } M _ { \sun } , derived from dynamical models ) are found to have H i masses up to M ( H i ) /M _ { \star } \sim 0.06 and H _ { 2 } masses up to M ( H _ { 2 } ) /M _ { \star } \sim 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 \leq 5 \times 10 ^ { 10 } M _ { \sun } , where such signatures are found in \sim 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 .