Mid- to Far-IR Emission and Star Formation in Early-Type Galaxies

TL;DR

This study uses Spitzer observations to explore star formation in early-type galaxies rich in molecular gas, finding that their infrared emission is largely due to star formation rather than AGN or evolved stars.

Contribution

It provides new evidence linking molecular gas to star formation activity in early-type galaxies through MIR to FIR observations.

Findings

24 micron emission correlates with star formation structures.

CO-rich galaxies show higher 24 micron luminosities than expected from evolved stars.

FIR/radio ratios support star formation as the emission source.

Abstract

Many early-type galaxies have been detected at wavelengths of 24 to 160 micron, but the emission is usually dominated by heating from an AGN 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 MIR to FIR emission could be associated with star formation activity. The 24 micron 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 micron emission can be traced to star formation with some notable exceptions. The 24 micron 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 the molecular gas is usually unstable to star formation activity.