The Fundamental Plane and the evolution of the M/L ratio of early-type field galaxies up to z~ 1

We analyse the Fundamental Plane (FP) of 26 strong gravitational lens galaxies with redshifts up to z∼ 1, using tabulated data from Kochanek et al. and Rusin et al. The lens galaxies effectively form a mass-selected sample of early-type galaxies in environments of relatively low density. We analyse the FP and its evolution in the rest-frame Johnson B- and Gunn r-bands. Assuming that early-type galaxies are a homologous family, the FP then provides a direct measurement of the M/L ratio evolution. If we assume that the M/L ratios of field early-type galaxies evolve as power laws, we find for the lens galaxies an evolution rate d log (M/L)/dz of −0.62 ± 0.13 in rest-frame B and −0.47 ± 0.11 in rest-frame Gunn r for a flat cosmology with ΩM= 0.3 and ΩΛ= 0.7. For a Salpeter initial mass function and solar metallicity these results correspond to mean stellar formation redshifts of 〈z⋆〉= 1.8+1.4−0.5 and 1.9+1.9−0.6 respectively. After correction for maximum progenitor bias, van Dokkum & Franx find a mean stellar formation redshift for cluster galaxies of 〈zcl⋆〉= 2.0+0.3−0.2, which is not significantly different from that found for the lens galaxies. However, if we impose the constraint that lens and cluster galaxies that are of the same age have equal M/L ratios and if we do not correct for progenitor bias, the difference is significant and we find that the stellar populations of the lens galaxies are 10–15 per cent younger than those of the cluster galaxies. We find that both the M/L ratios as well as the rest-frame colours of the lens galaxies show significant scatter. About half of the lens galaxies are consistent with an old cluster-like stellar population, but the other galaxies are bluer and fitted best by single burst models with younger stellar formation redshifts as low as z⋆∼ 1. Moreover, the scatter in colour is correlated with the scatter in M/L ratio. We interpret this as evidence of a significant age spread among the stellar populations of lens galaxies, whereas the ages of the stellar populations of the cluster galaxies are well approximated by a single formation epoch.