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

TL;DR

This study investigates the internal star formation and age gradients in z~1.5 quiescent galaxies, revealing evidence of outside-in rejuvenation or inside-out quenching, with low-level star formation and possible AGN activity.

Contribution

It provides the first spatially resolved spectroscopic analysis of quiescent galaxies at z~1.5, highlighting age gradients and residual star formation.

Findings

Outer regions are younger than cores by ~0.4 Gyr.

Detected low-level star formation rates around 3-4.5 Msun/yr.

Evidence of radio-mode AGN activity in the sample.

Abstract

We present a sample of 34 spectroscopically confirmed BzK-selected ~1e11 Msun 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 ~<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]3727A emission line, with SFR_OII = 3.7-4.5 Msun/yr. 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 Msun/yr). This second, photometric sample also displays significant excess at 1.4 GHz, suggestive of the presence of radio-mode AGN activity.