Morphological Studies of the Galaxy Populations in Distant “Butcher‐Oemler” Clusters with theHubble Space Telescope. II. AC 103, AC 118, and AC 114 atz= 0.31

Abstract

We present new results of a program to study the detailed morphologies of galaxies in intermediate-redshift clusters and hence understand the physical origin of the enhanced star formation seen in these environments at earlier epochs. Deep, high-resolution imagery has been obtained of three rich clusters, AC 103, AC 118, and AC 114 at z = 0.31, through the R (F702W) filter of the Wide Field Planetary Camera (WFPC2) of the Hubble Space Telescope (HST). For AC 103 and AC 118, single three-orbit pointings covering a central ~0.5 × 0.5 h^-1 Mpc area have been obtained. For AC 114, a mosaic of four six-orbit pointings has provided coverage of a central ~1.2 × 0.7 h^-1 Mpc area, for the first time allowing a study extending to the outer regions of a more distant cluster. Morphological classification has been conducted visually using these images, with Hubble types plus evidence of dynamical interactions and/or structural abnormalities recorded for all galaxies down to R₇₀₂ = 22.25 in AC 103 and AC 118 and R₇₀₂ = 23.00 in AC 114. New spectroscopy limited at K' ≤ 18 has also been acquired, providing within our WFPC2 images a total sample of 129 confirmed cluster members, of which 117 have secure star formation classifications (e.g., starburst, poststarburst, Hδ strong). Our study reveals that the mix of Hubble types within the distant clusters is, overall, significantly different from that seen in the same high-density environments nearby: there are proportionally more spiral galaxies and fewer E/S0 galaxies at these earlier epochs. Within the core (r 200 h^-1 kpc) regions of the three clusters, spirals covering the full Sa-Sdm/Irr range are present in numbers up to a factor of ~4 higher than that seen in present-day clusters, the highest fraction being observed in the least massive cluster within our sample, AC 103. Only in the virialized core of our most massive, regular cluster, AC 114, do we see morphological fractions approaching those of the present epoch. However, outside the core of this cluster the spiral fraction rises to ~60%: 3 times above the present-day level. Dynamical interactions are also widespread throughout the populations of all three clusters, with ~20% of the members, on average, showing morphological evidence of this phenomenon. The various subsets of "active" cluster members show emergent morphological trends. The majority of blue galaxies either undergoing a starburst or seen shortly (major mergers. These galaxies, however, are generally of modest luminosity (L ~ L* + 1 mag) even in this brightened phase; in their faded state they appear destined to become dwarfs, too faint to be included in magnitude-limited spectroscopic samples such as ours. Cluster members with ongoing star formation typical of present-day spirals are mostly normal Sb-Sdm/Irr Hubble types. Galaxies identified as having completed their last major episode of star formation 1-2 Gyr prior to the epoch of observation are conspicuous by their commonality in morphology, all being early-type (S0-Sb) disk systems. The red Hδ-strong objects, interpreted from their colors and spectra as being the remnants of secondary star formation in old dormant systems, have morphologies consistent with this picture, being a mixture of normal E and S0 galaxies. In combination, these results point to cluster galaxy evolution being driven by not one but several different physical processes. The only conspicuous one is galaxy-galaxy interactions and merging. However, our study reveals many galaxies where this process could not have been responsible for altering the course of their star formation activity. Furthermore, the normal disk morphology of the majority of these previously active galaxies indicates the operation of process(es) that halt star formation but leave the basic disk structure intact and largely unperturbed. This tends to favor mechanisms that affect the gas supply (e.g., ram-pressure stripping, galaxy infall) rather than processes like "galaxy harassment," where we simply do not see the population of severely distorted spirals expected if its operation is widespread.