DYNAMICAL MASSES OF EARLY-TYPE GALAXIES AT z ∼ 2: ARE THEY TRULY SUPERDENSE?

We measured stellar velocity dispersions sigma and derived dynamical masses of nine massive (M approximate to 10(11) M-circle dot) early-type galaxies (ETGs) from the Galaxy Mass Assembly ultra-deep Spectroscopic Survey (GMASS) sample at redshift 1.4 less than or similar to z less than or similar to 2.0. The s are based on individual spectra for two galaxies at z approximate to 1.4 and on a stacked spectrum for seven galaxies with 1.6 < z < 2.0, with 202 hr of exposure at the ESO Very Large Telescope. We constructed detailed axisymmetric dynamical models for the objects, based on the Jeans equations, taking the observed surface brightness (from deep HST/ACS observations), point-spread function, and slit effects into account. Our dynamical masses M-Jeans agree within less than or similar to 30% with virial estimates M-vir = 5 x R-e sigma(2)/G, although the latter tend to be smaller. Our MJeans also agrees within a factor less than or similar to 2 with the M-pop previously derived using stellar population models and 11 bands photometry. This confirms that the galaxies are intrinsically massive. The inferred mass-to-light ratios (M/L)(U) in the very age-sensitive rest-frame U band are consistent with passive evolution in the past similar to 1 Gyr (formation redshift z(f) similar to 3). A bottom-light stellar initial mass function appears to be required to ensure close agreement between M-Jeans and M-pop at z similar to 2, as it does at z similar to 0. The GMASS ETGs are on average more dense than their local counterpart. However, a few percent of local ETGs of similar dynamical masses also have comparable sigma and mass surface density Sigma(50) inside R-e.