The fundamental plane of field early-type galaxies at z=1

We present deep VLT spectra of early-type galaxies at z approximate to 1 in the Chandra Deep Field - South, from which we derive velocity dispersions. Together with structural parameters from Hubble Space Telescope imaging, we can study the fundamental plane for field early-type galaxies at that epoch. We determine accurate mass-to-light ratios (M/L) and colors for four field early-type galaxies in the redshift range, and two with 0.96 < z < 1.14. The galaxies were selected by color and morphology, and have generally red colors. Their 0.65 < z < 0.70 velocity dispersions show, however, that they have a considerable spread in M/L ( by a factor of 3). We find that the colors and directly measured M/L correlate well, demonstrating that the spread in M/L is real and reflects variations in stellar populations. The most massive galaxies have M/L comparable to massive cluster galaxies at similar redshift and therefore have stellar populations that formed at high redshift (). The lower mass galaxies z 1 2 at z approximate to 1 have a lower average M/L, and one is a genuine "E+A" galaxy. The M/L indicate that their luminosity-weighted ages are a factor of 3 younger at the epoch of observation, because of either a low formation redshift or late bursts of star formation contributing 20%- 30% of the mass.