MID- TO FAR-INFRARED EMISSION AND STAR FORMATION IN EARLY-TYPE GALAXIES

Many early-type galaxies have been detected at wavelengths of 24-160 mu m but the emission is usually dominated by heating from an active galactic nucleus or from the evolved stellar population. Here, we present Spitzer MIPS observations of a sample of elliptical and lenticular galaxies that are rich in cold molecular gas, and we investigate whether the mid-IR to far-IR (FIR) emission could be associated with star formation activity. The 24 mu m images show a rich variety of structures, including nuclear point sources, rings, disks, and smooth extended emission. Comparisons to matched-resolution CO and radio continuum images suggest that the bulk of the 24 mu m emission can be traced to star formation with some notable exceptions. The 24 mu m luminosities of the CO-rich galaxies are typically a factor of 15 larger than what would be expected from the dust associated with their evolved stars. In addition, FIR/radio flux density ratios are consistent with star formation. We conclude that the star formation rates in z = 0 elliptical and lenticular galaxies, as inferred by other authors from UV and optical data, are roughly consistent with the molecular gas abundances and that themolecular gas is usually unstable to star formation activity.