Molecular gas and star formation in the SAURON early-type galaxies

We present the results of a survey of CO emission in 43 of the 48 representative E/S0 galaxies observed in the optical with the SAURON integral-field spectrograph. The CO detection rate is 12/43 or 28 per cent. This is lower than previous studies of early-types but can probably be attributed to different sample selection criteria. As expected, earlier type, more luminous and massive galaxies have a relatively lower molecular gas content. We find that CO-rich galaxies tend to have higher H beta but lower Fe5015 and Mgb absorption indices than CO-poor galaxies. Those trends appear primarily driven by the age of the stars, a hypothesis supported by the fact that the galaxies with the strongest evidence of star formation are also the most CO-rich. In fact, the early-type galaxies from the current sample appear to extend the well-known correlations between far-infrared (FIR) luminosity, dust mass and molecular mass of other galaxy types. The star formation interpretation is also consistent with the SAURON galaxies' radio continuum and FIR flux ratios, and their inferred star formation efficiencies are similar to those in spiral galaxies. It thus appears that we have identified the material fueling (residual) star formation in early-type galaxies, and have demonstrated that it is actively being transformed. Nevertheless, the lack of strong correlations between the CO content and most stellar parameters is compatible with the idea that, in a significant number of sample galaxies, the molecular gas has been accreted from the outside and has properties rather independent from the old, pre-existing stellar component.