Spatially Resolved Stellar Kinematics of Field Early‐Type Galaxies at z = 1: Evolution of the Rotation Rate

Abstract

We use the spatial information of our previously published VLT/FORS2 absorption-line spectroscopy to measure mean stellar velocity and velocity dispersion profiles of 25 field early-type galaxies at a median redshift z = 0.97 (full range 0.6 < z < 1.2). This provides the first detailed study of early-type galaxy rotation at these redshifts. From surface brightness profiles from HST imaging we calculate two-integral oblate axisymmetric Jeans equation models for the observed kinematics. Fits to the data yield for each galaxy the degree of rotational support and the mass-to-light ratio M/LJeans. S0 and Sa galaxies are generally rotationally supported, whereas elliptical galaxies rotate less rapidly or not at all. Down to MB = ‑ 19.5 (corrected for luminosity evolution), we find no evidence for evolution in the fraction of rotating early-type (E+S0) galaxies between z ~ 1 (63% +/- 11% ) and the present (61% +/- 5% ). We interpret this as evidence for little or no change in the field S0 fraction with redshift. We compare M/LJeans with M/Lvir inferred from the virial theorem and globally averaged quantities and assuming homologous evolution. There is good agreement for nonrotating (mostly E) galaxies. However, for rotationally supported galaxies (mostly S0) M/LJeans is on average ~40% higher than M/Lvir. We discuss possible explanations and the implications for the evolution of M/L between z = 1 and the present and its dependence on mass. Based on observations collected at the European Southern Observatory, Chile (169.A-0458), and on observations with the Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.