Galaxy Morphologies in the Cluster Cl 1358+62 atz = 0.33

Abstract

We describe the morphological composition of a sample of 518 galaxies in the field of Cl 1358+62 at z = 0.33, drawn from a large Hubble Space Telescope mosaic covering 53 square arcminutes. The sample is complete to I = 22, corresponding to MV = -18.5 in the rest frame. The galaxy morphologies have been independently classified by the authors of this paper and by Alan Dressler. Dressler's classifications place our work in context with the previous MORPHS study and allow us to estimate the scatter between different sets of visual classifications. We restrict most of our analysis to the brighter part of the sample, I < 21 (MV < -19.5), where the scatter between the two sets of classifications is ~1 in morphological type. The scatter doubles at I = 22, presumably owing to the lower signal-to-noise and poorer sampling of faint, small galaxy images. To I = 21 the two sets of classifiers agree on the fraction of early type galaxies (elliptical + S0): 72%. We conclude that Cl 1358+62 does not contain the large population of spiral galaxies found in other studies of clusters at z ~ 0.3 and that there is probably a significant spread in the degree of cluster evolution at intermediate redshift. The two groups of classifiers differ on the relative fraction of S0 and elliptical galaxies. We show that the distributions of ellipticities and bulge/total light cannot resolve this discrepancy. Nonetheless, we can derive significant constraints on physical models for the evolution of the galaxy population in Cl 1358+62. The higher ratio of S0 to elliptical galaxies (1.6) found by DF/MF/PvD requires that the evolution preserve the relative fraction of elliptical, S0 and spiral galaxies. Alternately, the lower ratio (1.1) found by Dressler requires that the evolution preserve the early-type to spiral ratio while increasing the S0 to elliptical ratio. In the latter case, a possible evolutionary mechanism is accretion of galaxies that predominantly evolve to S0 galaxies between z = 0.33 and the present. We use our large body of spectra to make the correspondence between spectral and morphological type. Our data follow the pattern seen in the field at low redshift: emission-line spectra are more prevalent among the later morphological types. The 11 identified k+a galaxies (absorption-line spectra with strong Balmer lines) have S0-Sb morphologies.