EVOLUTION OF EARLY-TYPE GALAXIES IN DISTANT CLUSTERS - THE FUNDAMENTAL PLANE FROM HUBBLE-SPACE-TELESCOPE IMAGING AND KECK SPECTROSCOPY

We present new results on the fundamental plane of galaxies in two ric h clusters, Cl 1358+62 at z = 0.33 and MS 2053-04 at z = 0.58, based o n Keck and Hubble Space Telescope observations. Our new data triple th e sample of galaxies with measured fundamental plane parameters at int ermediate redshift. The early-type galaxies in these clusters define v ery clear fundamental plane relations, confirming an earlier result fo r Cl 0024+16, at z = 0.39. This large sample allows us to estimate the scatter reliably. We find it to be low, at +/-0.067 in log r(e), or 1 7% in r(e), similar to that observed in comparable low-redshift cluste rs. 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 clear ly evolve with redshift; the evolution is consistent with Delta log (M /L(V)) similar to -0.3z. The M/L(V) ratios of two E+A galaxies in Cl 1 358+62 are also lower by a factor of similar to 3, consistent with the hypothesis that they underwent a starburst 1 Gyr previously. We concl ude that the fundamental plane can therefore be used as a sensitive di agnostic of the evolutionary history of galaxies. Our data, when compa red with the predictions of simple stellar population models, imply th at the oldest cluster galaxies formed at high redshift (z > 2). We inf er a different evolutionary history for the E+A galaxies, in which a l arge fraction of stars formed at z < 1. Larger samples spanning a larg er redshift range are needed to determine the influence of starbursts on the general cluster population.