An ultra-deep near-infrared spectrum of a compact quiescent galaxy at
  z=2.2
  Several recent studies have shown that about half of the massive galaxies at
z~2 are in a quiescent phase. Moreover, these galaxies are commonly found to be
ultra-compact with half-light radii of ~1 kpc. We have obtained a ~29 hr
spectrum of a typical quiescent, ultra-dense galaxy at z=2.1865 with the Gemini
Near-Infrared Spectrograph. The spectrum exhibits a strong optical break and
several absorption features, which have not previously been detected in z>2
quiescent galaxies. Comparison of the spectral energy distribution with stellar
population synthesis models implies a low star formation rate (SFR) of 1-3
Msol/yr, an age of 1.3-2.2 Gyr, and a stellar mass of ~2x10^11 Msol. We detect
several faint emission lines, with emission-line ratios of [NII]/Halpha,
[SII]/Halpha and [OII]/[OIII] typical of low-ionization nuclear emission-line
regions. Thus, neither the stellar continuum nor the nebular emission implies
active star formation. The current SFR is <1% of the past average SFR. If this
galaxy is representative of compact quiescent galaxies beyond z=2, it implies
that quenching of star formation is extremely efficient and also indicates that
low luminosity active galactic nuclei (AGNs) could be common in these objects.
Nuclear emission is a potential concern for the size measurement. However, we
show that the AGN contributes <8% to the rest-frame optical emission. A
possible post-starburst population may affect size measurements more strongly;
although a 0.5 Gyr old stellar population can make up <10% of the total stellar
mass, it could account for up to ~40% of the optical light. Nevertheless, this
spectrum shows that this compact galaxy is dominated by an evolved stellar
population.
