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 ~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 FUV-NUV versus NUV-K color-color diagram with a Simple Stellar Population
(SSP) model shows that ellipticals and bright S0 galaxies are dominated by a
stellar population of age > 10^9 years while faint S0 galaxies may contain
stars as young as 10^8 years, providing evidence for relatively recent star
formation activity. The strength of the 4000A 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
characterised by an old coeval stellar population and classical bulge
