Luminosity Evolution of Field Early-Type Galaxies to [CLC][ITAL]z[/ITAL][/CLC] = 0.55

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Abstract
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/LB 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: ln M/LBfield - ln M/LBclus = -0.18 ± 0.11. The M/LB ratio of field early-type galaxies evolves as Δ 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 σ = 0.09 ± 0.02 in log re, 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. 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 Δ ln M/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.