Spatially resolved stellar kinematics of field early-type galaxies at z=1: Evolution of the rotation rate

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/L-Jeans. S0 and Sa galaxies are generally rotationally supported, whereas elliptical galaxies rotate less rapidly or not at all. Down to M-B - -19.5 (corrected for luminosity evolution), we find no evidence for evolution in the fraction of rotating early-type (E+S0) galaxies between z similar to 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/L-Jeans with M/L-vir 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/L-Jeans is on average similar to 40% higher than M/L-vir. We discuss possible explanations and the implications for the evolution of M/L between z = 1 and the present and its dependence on mass.