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 similar to 2 are in a quiescent phase. Moreover, these galaxies are commonly found to be ultra-compact with half-light radii of similar to 1 kpc. We have obtained a similar to 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 M(circle dot) yr(-1), an age of 1.3-2.2 Gyr, and a stellar mass of similar to 2 x 10(11) M(circle dot). We detect several faint emission lines, with emission-line ratios of [N II]/H alpha, [S II]/H alpha, and [O II]/[O III] 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 less than or similar to 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 less than or similar to 10% of the total stellar mass, it could account for up to similar to 40% of the optical light. Nevertheless, this spectrum shows that this compact galaxy is dominated by an evolved stellar population.