2002ApJ...564L..13T

We have measured the fundamental plane (FP) parameters for a sample of 30 field early-type galaxies (E/S0s) 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^+0.09_–0.13 (for Ω=0.3, Ω_Λ=0.7) or, in terms of average effective mass-to-light ratio, as dlog(M/L_B)/dz=-0.72^+0.11_–0.16, i.e., faster than is observed for cluster E/S0s (-0.49±0.05). In addition, we detect [O II] emission greater than 5 Å in 22% of an enlarged sample of 42 massive E/S0s 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/S0s experiences secondary episodes of star formation at z<1.