A Wide-Field Hubble Space Telescope Survey of the Cluster Cl 0024+16 at z = 0.4. III. Spectroscopic Signatures of Environmental Evolution in Early-Type Galaxies

We report results from a panoramic spectroscopic survey of 955 objects in the field of the rich cluster Cl 0024+1654 (z~=0.4), complementing the HST imaging presented in the first paper in this series. Combining with previous work, we compile a catalog of 1394 unique redshifts in the field of this cluster, including 486 cluster members spread across an area 10 Mpc in diameter. Our new spectroscopic sample includes over 200 high-quality spectra of cluster members. We examine the properties of a large sample of 104 cluster early-type galaxies as a function of cluster radius and local density, using them as sensitive tracers of the various physical processes that may be responsible for galaxy evolution. By constructing the fundamental plane of Cl 0024, we infer an evolution in the mean mass-to-light ratio of early-type galaxies with respect to z=0 of Δ<log(M/L_V)>=-0.14+/-0.02. In the cluster center, we detect a significantly increased scatter in the relationship compared to that seen in local clusters. Moreover, we observe a clear radial trend in the mass-to-light ratios of individual early-type galaxies, with the oldest galaxies located in the cluster core. Galaxies are apparently younger at larger radius, with E+S0 galaxies in the periphery having M/L_V ratios that nearly match values seen in the field at a similar redshift. The strong radial trend is seen even when the sample is restricted to a narrow range in galaxy mass. Independent spectral indicators used in combination reveal an abrupt interaction with the cluster environment that occurs near the virial radius of Cl 0024, revealed by small bursts of star formation in a population of dim early-type galaxies, as well as by enhanced Balmer absorption for a set of larger E+S0 galaxies closer to the cluster core. We construct a simple infall model used to compare the timescales and strengths of the observed interactions in this cluster. We examine the possibility that bursts of star formation are triggered when galaxies suffer shocks as they encounter the intracluster medium, or by the onset of galaxy harassment.