The evolution of field early-type galaxies to z ∼ 0.7

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 gamma evolves as dgamma/dz = 0.58(-0.13)(+0.09) (for Omega=0.3, Omega(Lambda)=0.7) or, in terms of average effective mass-to-light ratio, as d log (M/L-B)/dz = -0.72(-0.16)(+0.11) , i. e., faster than is observed for cluster E/S0s (-0.49+/-0.05). In addition, we detect [O II] emission greater than 5 Angstrom 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.