Stellar population trends in S0 galaxies

We present stellar population age and metallicity trends for a sample of 59 S0 galaxies based on optical Sloan Digital Sky Survey (SDSS) and near-infrared (NIR) J and H photometry. When combined with optical g and r passband imaging data from the SDSS archive and stellar population models, we obtain radial age and metallicity trends out to at least five effective radii for most of the galaxies in our sample. The sample covers a range in stellar mass and light concentration. We find an average central light-weighted age of ∼4 Gyr and central metallicity [Z/H]∼ 0.2 dex. Almost all galaxies show a negative metallicity gradient from the centre out, with an average value of Δ[Z/H]/Δlog (r/R_e) =-0.6. An age increase, decrease and minimal change with radius is observed for 58 per cent, 19 per cent and 23 per cent, respectively, for a mean age gradient of Δage/Δlog (r/R_e) = 2.3 Gyr dex^-1. For 14 out of 59 galaxies, the light-weighted age of the outer region is greater than 10 Gyr. We find that galaxies with both lower mass and lower concentration have younger light-weighted ages and lower light-weighted metallicities. This mass-metallicity relation extends into the outer regions of our S0 galaxies. Our results are consistent with the formation of S0 galaxies through the transformation of spiral galaxy discs. Determining the structural component that makes up the outer region of galaxies with old outskirts is a necessary step to understand the formation history of S0 galaxies.