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Fundamental Plane studies provide an excellent means of understanding theevolutionary history of early-type galaxies. Using the Low Resolution ImagingSpectrograph on the Keck telescope, we obtained internal stellar kinematicinformation for 36 field galaxies in the Groth Strip--21 early-type and 15 diskgalaxies. Their redshifts range from 0.3--1.0, with a median redshift 0.8. Theslope of the relation shows no difference compared with the local slope.However, there is significant evolution in the zero-point offset; an offset dueto evolution in magnitude requires a 2.4 magnitude luminosity brightening atz=1. We see little differences of the offset with bulge fraction, which is agood surrogate for galaxy type. Correcting for the luminosity evolution reducesthe orthogonal scatter in the Fundamental Plane to 8%, consistent with thelocal scatter. This scatter is measured for our sample, and does not includeresults from other studies which may have different selection effects. Thedifference in the degree of evolution between our field sample and publishedcluster galaxies suggests a more recent formation epoch--around z=1.5 for fieldgalaxies compared to z>2.0 for cluster galaxies. The magnitude differenceimplies that the field early-type galaxies are about 2 Gyr younger than thecluster ellipticals using standard single-burst models. However, the samemodels imply a significant change in the rest-frame U-B color from then topresent, which is not seen in our sample. Continuous low-level star formation,however, would serve to explain the constant colors over this large magnitudechange. A consistent model has 7% of the stellar mass created after the initialburst, using an exponentially decaying star formation rate with an e-foldingtime of 5 Gyr.

The DEEP Groth Strip Survey. IX. Evolution of the Fundamental Plane of Field Galaxies