Stellar population trends in S0 galaxies

We present stellar population age and metallicity trends for a sample of fifty-nine S0 galaxies based on optical SDSS and NIR J & H photometry. When combined with optical g and r passband imaging data from the SDSS archive and stellar population models, we calculate radial age and metallicity gradients out to at least 5 effective radii for most of the galaxies in our sample. The sample covers a range in stellar mass, light concentration, and environmental density. We find an average central light-weighted age of ∼3 Gyr and central metallicity [Z/H]∼0.5 dex. Almost all galaxies show a negative metallicity gradient from the center out, with an average value of Delta[Z/H]/Deltalog(r) = -0.5. We observe an increase in age with radius for 41% of our sample, a decrease for 17%, and small change for 42%. Galaxies with both lower mass and lower concentration have younger light-weighted ages than other galaxies in our sample. For 20% of our sample, the light-weighted ages of the outer regions are greater than 10 Gyr. In order to understand if the old regions of these galaxies are dominated by a disk component, we have performed galactic component decompositions of a sub-sample of 22 S0 galaxies. The sub-sample focuses on the S0 galaxies with a substantially old outer regions and includes additional galaxies for comparison. Our decomposition routine uses a generalized Sersic component for the bulge and an exponential profile for the disk. Nearly all galaxies that were decomposed show outer regions that are disk dominated. Our results indicate that the disk component is responsible for the old ages in the outer regions. The ages of the disks of these galaxies place a constraint on models of hierarchical merging, requiring no major merger to have occurred for these galaxies in a very long time (since z ∼ 2, using an age of 10 Gyr). For the sub-sample of galaxies that we analyze with profile decompositions, we derive a mean n of 2.0 +/- 0.6, a mean ratio of bulge and disk scale lengths, re/h, of 0.41 +/- 0.40, and a mean (B/D)tot of 1.8 +/- 3.1. We find a correlation between n and (B/D)tot such that galaxies with larger n are more bulge dominated.