Hubble Space Telescope Observations of the Distant Cluster CL 0016+16: Quantitative Morphology of Confirmed Cluster Members

Abstract

We present Hubble Space Telescope (HST) images of 24 confirmed members of the distant galaxy cluster Cl 0016+16 at redshift 0.55. The Balmer- strong ("E + A") and emission-line galaxies frequently show unusual visual morphology, implying that galaxian interactions produce "active" galaxies in moderate-redshift clusters. We use an image concentration index as a quantitative measure of morphology to show that these unusual galaxies appear disklike, while the normal red galaxies resemble E/S0 galaxies. Although consistent with HST observations by Dressler et al. in Cl 0939+4713 (z = 0.41), our results differ from the Couch et al. finding that most Balmer-strong galaxies in AC 114 (z = 0.31) resemble ellipticals. The entire "E + A" sample is small, but if future studies confirm their diversity, it will suggest that they have different origins.