The Atlas3D Project - XXX. Star formation histories and stellar
  population scaling relations of early-type galaxies
  We present the stellar population content of early-type galaxies from the
Atlas3D survey. Using spectra integrated within apertures covering up to one
effective radius, we apply two methods: one based on measuring line-strength
indices and applying single stellar population (SSP) models to derive
SSP-equivalent values of stellar age, metallicity, and alpha enhancement; and
one based on spectral fitting to derive non-parametric star-formation
histories, mass-weighted average values of age, metallicity, and half-mass
formation timescales. Using homogeneously derived effective radii and
dynamically-determined galaxy masses, we present the distribution of stellar
population parameters on the Mass Plane (M_JAM, Sigma_e, R_maj), showing that
at fixed mass, compact early-type galaxies are on average older, more
metal-rich, and more alpha-enhanced than their larger counterparts.
  From non-parametric star-formation histories, we find that the duration of
star formation is systematically more extended in lower mass objects. Assuming
that our sample represents most of the stellar content of today's local
Universe, approximately 50% of all stars formed within the first 2 Gyr
following the big bang. Most of these stars reside today in the most massive
galaxies (>10^10.5 M_sun), which themselves formed 90% of their stars by z~2.
The lower-mass objects, in contrast, have formed barely half their stars in
this time interval. Stellar population properties are independent of
environment over two orders of magnitude in local density, varying only with
galaxy mass. In the highest-density regions of our volume (dominated by the
Virgo cluster), galaxies are older, alpha-enhanced and have shorter
star-formation histories with respect to lower density regions.
