Luminosity evolution of field early-type galaxies to z=0.55

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 MIL ratios of field and cluster early-type galaxies at intermediate redshift is small and not significant: [ln M/L-B](field) - [ln M/L-B](clus) = -0.18 +/- 0.11. The M/L-B ratio of field early-type galaxies evolves as Delta ln M/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 sigma = 0.09 +/- 0.02 in log r(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 a = 0.03(-0.03)(+0.04). 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 MIL ratios of cluster and field early-type galaxies provides a constraint on the relative ages of their stars. At (z) = 0.43, held 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 greater than or equal to 1.5. Recent semianalytical models for galaxy formation in a Lambda CDM universe predict a systematic difference between held and cluster galaxies of Delta ln M/L-B similar to -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.