Mass-to-light ratios of field early-type galaxies at z ∼ 1 from ultradeep spectroscopy:: Evidence for mass-dependent evolution

We present an analysis of the fundamental plane for a sample of 27 field early-type galaxies in the redshift range 0.6 < z < 1.15 in the Chandra Deep Field-South and the field of the background cluster RDCS 1252.9-2927. Sixteen of the galaxies are at z > 0.95. The galaxies in this sample have high signal-to-noise ratio spectra obtained at the Very Large Telescope and high-resolution imaging from the HST Advanced Camera for Surveys. From comparison with lower redshift data, we find that the mean evolution of the mass-to-light ratio (M/L) of our sample is Delta 1n (M/LB) = (-1.74 +/- 0.16) z, with a large galaxy-to-galaxy scatter. The strong correlation between M/L and rest-frame color indicates that the observed scatter is not due to measurement errors, but due to intrinsic differences between the stellar populations of the galaxies, such that our results can be used as a calibration for converting luminosities of high-redshift galaxies into masses. This pace of evolution is much faster than the evolution of cluster galaxies. However, we find that the measured M/L evolution strongly depends on galaxy mass. For galaxies with masses M > 2 x 10(11) M circle dot, we find no significant difference between the evolution of field and cluster galaxies: Delta 1n (M/L-B) = (-1.20 +/- 0.18) z for field galaxies and Delta 1n (M/L-B) (-1.12 +/- 0.06) z for cluster galaxies. The relation between the measured M/ L evolution and mass is partially due to selection effects, as the galaxies are selected by luminosity, not mass. We calculate the magnitude of this effect for the subsample of galaxies with masses higher than M = 6 x 10(10) M circle dot: the uncorrected value of the evolution is Delta 1n (M/LB) (-1.54 +/- 0.16) z, whereas the corrected value is (-1.43 +/- 0.16) z. However, even when taking selection effects into account, we still find a relation between M/ L evolution and mass, which is most likely caused by a lower mean age and a larger intrinsic scatter for low-mass galaxies. Results from lensing early-type galaxies, which are mass selected, show a very similar trend with mass. This, combined with our findings, provides evidence for downsizing, i.e., for the proposition that low-mass galaxies are younger than high-mass galaxies. Previous studies of the rate of evolution of field early-type galaxies found a large range of mutually exclusive values. We show that these differences are largely caused by the differences between fitting methods: most literature studies are consistent with our result and with one another when using the same method. Finally, five of the early-type galaxies in our sample have AGNs. There is tentative evidence that the stellar populations in these galaxies are younger than those of galaxies without AGNs.