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 HST WFPC2 observations and deep Keck spectroscopy. Structural parameters and internal velocity dispersions are measured for eighteen galaxies at 0.15 < z < 0.55. Rest frame M⁄ LB ratios are determined from the Fundamental Plane 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⁄ LB〉field − 〈lnM⁄ LB〉clus = −0.18±0.11. The M⁄ LB ratios of field early-type galaxies evolve as ∆ ln M⁄ LB = (−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 σBI = 0.09±0.02 in logre, 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 semi-analytical models for galaxy formation in a Λ-CDM Universe predict a systematic difference between field and cluster galaxies of ∆ lnM⁄ LB ∼ −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. Subject headings: galaxies: evolution, galaxies: elliptical and lenticular, cD, galaxies: structure of