Evidence of Star Formation in Local S0 Galaxies: Spitzer Observations of the Sauron Sample

We discuss infrared Spitzer observations of early-type galaxies in the SAURON sample at 24, 60, and 170 μm. When compared with 2MASS Ks band luminosities, lenticular (S0) galaxies exhibit a much wider range of mid-to-far-infrared luminosities than elliptical (E) galaxies. Mid- and far-infrared emission from E galaxies is a combination of circumstellar or interstellar emission from local mass-losing red giant stars, dust buoyantly transported from the galactic cores into distant hot interstellar gas and dust accreted from the environment. The source of mid- and far-IR emission in S0 galaxies is quite different and is consistent with low levels of star formation, 0.02-0.2 M sun yr-1, in cold, dusty gaseous disks. The infrared 24 μm-70 μm color is systematically lower for (mostly S0) galaxies with known molecular disks. Our observations support the conjecture that cold dusty gas in some S0 galaxies is created by stellar mass loss at approximately the same rate that it is consumed by star formation, so the mass depletion of these disks by star formation will be slow. Unlike E galaxies, the infrared luminosities of S0 galaxies correlate with both the mass of molecular gas and the stellar Hβ spectral index, and all are related to the recent star formation rate (SFR). However, star formation rates estimated from the Hβ-emission-line luminosities L Hβ in SAURON S0 galaxies are generally much smaller. Since L Hβ does not correlate with 24 μm emission from dust heated by young stars, optical emission lines appear to be a poor indicator of SFRs in SAURON S0 galaxies. The absence of Hβ emission may be due to a relative absence of OB stars in the initial mass function or to dust absorption of Hβ emission lines.