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