THE MEAN AGES OF S0 DISKS - EVIDENCE FOR STAR FORMATION 5 GIGAYEARS AGO

We have acquired optical and infrared photometry of ≈35 S0 disks and bulges. Using B - H and J - K colors allows for an approximate separation of the effects of age from metallicity in determining the colors of a stellar population. True disk colors are derived from a decomposition of the surface brightness profile into bulge and disk components. In terms of structural properties, we find that the disks of S0s are very similar in surface brightness to spiral disks but, on average, S0s have smaller scale lengths, which is reflected in their generally large bulge-to-disk ratios. For our sample, the bulge contributes ≈67% of the total luminosity. In terms of photometric properties, we find that S0 disks and bulges are well separated in the B - H versus J - K diagram, indicating that a different stellar population is present in the disk compared to the bulge. The similarity of disk and bulge J - K and H - K colors argues for a similarity in metallicity, and hence the best candidate to explain the observed average displacement of 0.5 mag in B - H is a lower mean age for the disk compared to the bulge. Comparison to model colors suggests that the disks are 3-5 Gyr younger than the bulges. In some extreme cases, we find evidence of AGB light indicating that star formation was active as little as 2-3 Gyr ago. This extended period of formation for S0 disks provides the most natural explanation for the prevalence of blue galaxies observed in intermediate redshift clusters of galaxies. The preponderance of S0 galaxies in nearby clusters indicates that they probably were born in that environment and experienced a star formation history that was either terminated by astration or by the process of cluster virialization.