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 similar to 1.3. We derive galaxy properties (stellar masses, ages, and star formation histories) for samples of massive, red, passive early-type galaxies (ETGs) 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- to 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 ETGs. However, when comparing cluster versus field stellar population properties we find that, even if the 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 1Gyr 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.