2001ApJ...553L..39V

We study the fundamental plane (FP) of field early-type galaxies at intermediate redshift, using Hubble Space Telescope Wide Field Planetary Camera 2 observations and deep Keck spectroscopy. Structural parameters and internal velocity dispersions are measured for 18 galaxies at 0.15<z<0.55. Rest-frame M/L_B ratios are determined from the FP and compared to those of cluster early-type galaxies at the same redshifts. The systematic offset between M/L ratios of field and cluster early-type galaxies at intermediate redshift is small and not significant: <lnM/L_B>_field-<lnM/L_B>_clus=-0.18±0.11. The M/L_B ratio of field early-type galaxies evolves as ΔlnM/L_B=(-1.35±0.35)z, very similar to cluster early-type galaxies. After correcting for luminosity evolution, the FP of field early-type galaxies has a scatter σ=0.09±0.02 in logr_e, similar to that in local clusters. The scatter appears to be driven by low-mass S0 galaxies; for the elliptical galaxies alone we find σ=0.03^+0.04_–0.03. There is a hint that the FP has a different slope than in clusters, but more data are needed to confirm this. The similarity of the M/L ratios of cluster and field early-type galaxies provides a constraint on the relative ages of their stars. At <z≥0.43, field early-type galaxies are younger than cluster early-type galaxies by only 21%±13%, and we infer that the stars in field early-type galaxies probably formed at z≳1.5. Recent semianalytical models for galaxy formation in a ΛCDM universe predict a systematic difference between field and cluster galaxies of ΔlnM/L_B~-0.6, much larger than the observed difference. This result is consistent with the hypothesis that field early-type galaxies formed earlier than predicted by these models.