Recent star formation in quiescent z~1 galaxies

The classical early/late-type galaxy dichotomy is increasingly less well-defined at higher redshift, when different epochs of galaxy assembly and evolution are probed than in the local universe. Using new rest-frame UV imaging obtained by the UVCANDELS HST Treasury Program in CANDELS deep fields (GOODS-N, GOODS-S, EGS, and COSMOS) we identify a sample (N~400) 0.5<z<1.5 massive (log(M)~10.5) quiescent galaxies. We combine the new UV photometry with archival imaging and spectroscopy to measure the size evolution of these galaxies, their star formation histories, and the relative richness of their local environments. We find a size growth, at a fixed stellar mass, of ~2x over the redshift range probed, in agreement with previous surveys at this redshift. Though these galaxies were selected as quiescent (i.e., effectively devoid of significant recent star formation) using UVJ color-color criterion, the long baseline in UV wavelength coverage now available for these galaxies reveals that ~15% of this sample experienced a burst of recent star-formation (t<1Gya, M_burst/M_stellar~10^-3). In addition, we find a weak statistical correlation between the richness of the local environment for quiescent galaxies and the presence of young stars. We interpret these observed trends as an indication that the establishment of the quiescent galaxy population is relatively extended in time (from z~4 to the current epoch) and is punctuated by periodic wet, minor mergers. These mergers act to “grow” early-type galaxies from their compact sizes at the formation redshift, while delivering cold gas which may lead to a burst of star-formation, the characteristics of which can be uniquely studied with the UVCANDELS survey imaging at z~1.