Early-Type galaxies at z ~ 1.3. III. On the dependence of Formation
  Epochs and Star Formation Histories on Stellar Mass and Environment
  We study the environmental dependence of stellar population properties at z ~
1.3. We derive galaxy properties (stellar masses, ages and star formation
histories) for samples of massive, red, passive early-type galaxies in two
high-redshift clusters, RXJ0849+4452 and RXJ0848+4453 (with redshifts of z =
1.26 and 1.27, respectively), and compare them with those measured for the
RDCS1252.9-2927 cluster at z=1.24 and with those measured for a similarly
mass-selected sample of field contemporaries drawn from the GOODS-South Field.
Robust estimates of the aforementioned parameters have been obtained by
comparing a large grid of composite stellar population models with extensive
8-10 band photometric coverage, from the rest-frame far-ultraviolet to the
infrared. We find no variations of the overall stellar population properties
among the different samples of cluster early-type galaxies. However, when
comparing cluster versus field stellar population properties we find that, even
if the (star formation weighted) ages are similar and depend only on galaxy
mass, the ones in the field do employ longer timescales to assemble their final
mass. We find that, approximately 1 Gyr after the onset of star formation, the
majority (75%) of cluster galaxies have already assembled most (> 80%) of their
final mass, while, by the same time, fewer (35%) field ETGs have. Thus we
conclude that while galaxy mass regulates the timing of galaxy formation, the
environment regulates the timescale of their star formation histories.
