The Evolution of Field Early - Type Galaxies to Z ∼ 0

We have measured the Fundamental Plane (FP) parameters for a sample of 30 field early-type galaxies (E/S0) in the redshift range 0.1 < z < 0.66. We find that: i) the FP is defined and tight out to the highest redshift bin; ii) the intercept γ evolves as dγ/dz = 0.58 −0.13 (for Ω = 0.3,ΩΛ = 0.7), or, in terms of average effective mass to light ratio, as d log(M/LB)/dz = −0.72 +0.11 −0.16, i. e. faster than is observed for cluster E/S0 (−0.49± 0.05). In addition, we detect [OII] emission > 5Å in 22% of an enlarged sample of 42 massive E/S0 in the range 0.1 < z < 0.73, in contrast with the quiescent population observed in clusters at similar z. We interpret these findings as evidence that a significant fraction of massive field E/S0 experiences secondary episodes of star-formation at z < 1. Subject headings: galaxies: elliptical and lenticular, cD — galaxies: evolution —galaxies: formation — galaxies: structure