ABSTRACT:
The most effective probe of the physical nature of quiescent galaxies is absorption line spectroscopy, which is particularly challenging at high redshift. Using the improved sensitivity of optical and infrared detectors at the Keck observatory, and the multiplex advantage of its new MOSFIRE spectrograph, we have undertaken a new spectroscopic survey in the redshift range 1 < z < 2.5. Velocity dispersions and stellar ages derived from our spectra, together with HST-based sizes, provide valuable insight into the mass assembly of quiescent galaxies. We find that the stellar to dynamical mass ratio evolves with redshift, which might imply a change in the dark matter fraction or in the stellar initial mass function. Our main conclusion is that the population of compact quiescent galaxies at high redshift grows in size partly via minor mergers (physical growth) and partly because of the increasing contribution of recently quenched, larger galaxies (progenitor bias). Finally, by fitting stellar population models to the spectroscopic and photometric data, we are able to robustly constrain, for the first time, the mass assembly and star formation histories of z > 2 quiescent systems.