In and out star formation in z?~?1.5 quiescent galaxies from rest-frame UV spectroscopy and the far-infrared

摘要

We present a sample of 34 spectroscopically confirmed BzK -selected ~ 10~(11) M _(⊙) quiescent galaxies (pBzK) in the COSMOS field. The targets were initially observed with VIMOS on the VLT to facilitate the calibration of the photometric redshifts of massive galaxies at z ? 1.5 . Here we describe the reduction and analysis of the data, and the spectrophotometric properties of these pBzK galaxies. In particular, using a spatially resolved median 2D spectrum, we find that the fraction of stellar populations with ages & 1 Gyr is at least 3 times higher in the outer regions of the pBzK galaxies than in their cores. This results in a mild age gradient of Δage ≤ 0.4 Gyr over ~ 6 kpc and suggests either the occurrence of widespread rejuvenation episodes or that inside-out quenching played a role in the passivization of this galaxy population. We also report on low-level star formation rates derived from the [OII]3727 ? emission line, with SFR _(OII) ~ 3.7?4.5 M _(⊙) yr ~(-1) . This estimate is confirmed by an independent measurement on a separate sample of similarly-selected quiescent galaxies in the COSMOS field, using stacked far-infrared data ( SFR _(FIR) ~ 2?4 M _(⊙) yr ~(-1) ). This second, photometric sample also displays significant excess at 1.4 GHz, suggestive of the presence of radio-mode AGN activity.