SDSS-IV MaNGA: The Formation Sequence of S0 Galaxies
  Gas stripping of spiral galaxies or mergers are thought to be the formation
mechanisms of lenticular galaxies. In order to determine the conditions in
which each scenario dominates, we derive stellar populations of both the bulge
and disk regions of 279 lenticular galaxies in the MaNGA survey. We find a
clear bimodality in stellar age and metallicity within the population of S0s
and this is strongly correlated with stellar mass. Old and metal-rich bulges
and disks belong to massive galaxies, and young and metal-poor bulges and disks
are hosted by low-mass galaxies. From this we conclude that the bulges and
disks are co-evolving. When the bulge and disk stellar ages are compared, we
find that the bulge is almost always older than the disk for massive galaxies
($\textrm{M}_{\star} > 10^{10}~\textrm{M}_{\odot}$). The opposite is true for
lower mass galaxies. We conclude that we see two separate populations of
lenticular galaxies. The old, massive, and metal-rich population possess bulges
that are predominantly older than their disks, which we speculate may have been
caused by morphological or inside-out quenching. In contrast, the less massive
and more metal-poor population have bulges with more recent star formation than
their disks. We postulate they may be undergoing bulge rejuvenation (or disk
fading), or compaction. Environment doesn't play a distinct role in the
properties of either population. Our findings give weight to the notion that
while the faded spiral scenario likely formed low-mass S0s, other processes,
such as mergers, may be responsible for high-mass S0s.
