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.

6 pages, 4 figures, LaTeX with emulateapj. Accepted for publication in ApJ Letters

References in corpus (11)

• Measuring the inclination and mass-to-light ratio of axisymmetric galaxies via anisotropic Jeans models of stellar kinematics
• Size evolution of the most massive galaxies at 1.7<z<3 from GOODS NICMOS survey imaging
• The Gemini Deep Deep Survey: III. The abundance of massive galaxies 3-6 billion years after the Big Bang
• NICMOS Imaging of DRGs in the HDF-S: A Relation Between Star-Formation and Size at z ~ 2.5
• Superdense massive galaxies in the Nearby Universe
• The Kormendy relation of massive elliptical galaxies at z~1.5. Evidence for size evolution ?
• The redshift evolution of early-type galaxies in COSMOS: Do massive early-type galaxies form by dry mergers?
• Evolution in the structural properties of early-type Brightest Cluster Galaxies at small lookback time and dependence on the environment
• Testing the Accuracy of Synthetic Stellar Libraries
• Spatially Resolved Stellar Kinematics of Field Early-Type Galaxies at z=1: Evolution of the Rotation Rate
• Dynamical Models of Elliptical Galaxies in z=0.5 Clusters: I. Data-Model Comparison and Evolution of Galaxy Rotation