Dynamical masses of early-type galaxies at z~2: Are they truly superdense?

Abstract

We measured stellar velocity dispersions sigma and dynamical masses of 9 massive (M~10^11 Msun) early-type galaxies (ETG) from the GMASS sample at redshift 1.4<z<2.0. The sigma are based on individual spectra for two galaxies at z~1.4 and on a stacked spectrum for 7 galaxies with 1.6<z<2.0, with 202-h 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), PSF and slit effects into account. Our dynamical masses M_Jeans agree within ~30% with virial estimates M_vir=5*Re*sigma^2/G, although the latter tend to be smaller. This suggests that sizes are not underestimated by more than a similar fraction. Our M_Jeans also agrees within a factor <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 ~1 Gyr (formation redshift z_f~3). A 'bottom-light' stellar Initial Mass Function (IMF) appears to be required to ensure close agreement between M_Jeans and M_pop at z~2, as it does at z~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 Re.