Luminosity-dependent star formation history of S0 galaxies: evidence from GALEX-SDSS-2MASS-WISE colours

We combine UV/optical/near-IR/mid-IR data on a sample of similar to 240 S0 galaxies to examine various star formation related processes in them. We split the sample into bright and faint S0 galaxies based on their K-band luminosity. Comparing the far-ultraviolet (FUV)-near-ultraviolet (NUV) versus NUV-K colour-colour diagram with a simple stellar population model shows that ellipticals and bright S0 galaxies are dominated by a stellar population of age >10(9) yr while faint S0 galaxies may contain stars as young as 10(8) yr, providing evidence for relatively recent star formation activity. The strength of the 4000 angstrom break is also systematically higher in brighter S0 galaxies, again indicating the presence of an old stellar population. Their mid-IR colours indicate that bright S0 colours are like those of ellipticals while faint S0 colours are more like spirals. All these observations are consistent with a scenario in which low-luminosity S0 galaxies likely formed by the stripping of gas from the discs of late-type spiral galaxies, which in turn formed their pseudo-bulges through secular evolution processes, possibly involving multiple episodes of star formation. On the other hand, more luminous S0 galaxies likely formed the bulk of their stars at early epochs, similar to the star formation in elliptical galaxies, and are characterized by an old coeval stellar population and classical bulges.