Evolution of Early-Type Galaxies in Distant Clusters: The Fundamental Plane using HST Imaging and Keck Spectroscopy

We present new results on the Fundamental Plane of galaxies in two rich clusters, Cl1358+62 at z=0.33 and MS2053-04 at z=0.58, based on Keck and HST observations. Our new data triple the sample of galaxies with measured Fundamental Plane parameters at intermediate redshift. The early-type galaxies in these clusters define very clear Fundamental Plane relations, confirming an earlier result for Cl0024+16 at z=0.39. This large sample allows us to estimate the scatter reliably. We find it to be low, at 0.067 dex in $log r_e$, or 17% in re, similar to that observed in comparable low redshift clusters. This suggests that the structure of the older galaxies has changed little since z=0.58. The M/L_V ratios of early-type galaxies clearly evolve with redshift; the evolution is consistent with $\Delta \log M/L_V \sim -0.3z$. The M/L_V ratios of two E+A galaxies in Cl1358+62 are also lower by a factor of $\sim 3$, consistent with the hypothesis that they underwent a starburst 1 Gyr previously. We conclude that the Fundamental Plane can therefore be used as a sensitive diagnostic of the evolutionary history of galaxies. Our data, when compared to the predictions of simple stellar population models, imply that the oldest cluster galaxies formed at high redshift (z > 2). We infer a different evolutionary history for the E+A galaxies, in which a large fraction of stars formed at z<1. Larger samples spanning a larger redshift range are needed to determine the influence of starbursts on the general cluster population.