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 M⊙ yr-1, an age of 1.3-2.2 Gyr, and a stellar mass of ∼ 2 × 1011 M⊙. We detect several…