Ubiquitous [OII] Emission in Quiescent Galaxies at z ~ 0.85

TL;DR

This study reveals that a significant majority of quiescent galaxies at z~0.85 exhibit ionized gas traced by [OII] emission, challenging the traditional view of their inactivity and suggesting ongoing processes like low-level star formation or AGN activity.

Contribution

It provides the first systematic evidence of widespread [OII] emission in quiescent galaxies at high redshift using deep spectroscopy from the LEGA-C survey.

Findings

59% of UVJ-quiescent galaxies show [OII] emission.

Higher [OII] luminosities per stellar mass compared to low-redshift samples.

Ubiquitous [OII] emission indicates ongoing processes in quiescent galaxies at z~0.85.

Abstract

Using deep rest-frame optical spectroscopy from the Large Early Galaxy Astrophysical Census (LEGA-C) survey, conducted using VIMOS on the ESO Very Large Telescope, we systematically search for low-ionization [OII] 3726,3729 emission in the spectra of a mass-complete sample of z~0.85 galaxies. Intriguingly, we find that 59 percent of UVJ-quiescent (i.e. non star-forming) galaxies in the sample have ionized gas, as traced by [OII] emission, detected above our completeness limit of 1.5 Angstroms. The median stacked spectrum of the lowest equivalent width quiescent galaxies also shows [OII] emission. The overall fraction of sources with [OII] above our equivalent width limit is comparable to what we find in the low-redshift Universe from GAMA and MASSIVE, except perhaps at the highest stellar masses (log Mstar/Msol > 11.5). However, stacked spectra for the individual low-equivalent width systems uniquely indicates ubiquitous [OII] emission in the higher-z LEGA-C sample, with typical [OII] luminosities per unit stellar mass that are a factor of 3 larger than the lower-z GAMA sample. Star formation in these otherwise quiescent galaxies could play a role in producing the [OII] emission at higher-z, although it is unlikely to provide the bulk of the ionizing photons. More work is required to fully quantify the contributions of evolved stellar populations or active galactic nuclei to the observed spectra.