Distinguishing Local and Global Influences on Galaxy Morphology: A Hubble Space Telescope Comparison of High and Low X-Ray Luminosity Clusters

We present a morphological analysis of 17 X-ray-selected clusters at z ~ 0.25, imaged uniformly with the Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2). Eight of these clusters comprise a subsample selected for their low X-ray luminosities (≲1044 ergs s-1), called the low-LX sample. The remaining nine clusters comprise a high-LX subsample with LX > 1045 ergs s-1. The two subsamples differ in their mean X-ray luminosity by a factor of 30 and span a range of more than 300. The clusters cover a relatively small range in redshift (z = 0.17-0.3, σz/z ~ 0.15), and the data are homogeneous in terms of depth, resolution (0.″17 = 1 h kpc at z = 0.25), and rest wavelength observed, minimizing differential corrections from cluster to cluster. We fit the two-dimensional surface brightness profiles of galaxies down to very faint absolute magnitudes, M702 ≤ -18.2 + 5 log h50 (roughly 0.01L) with parametric models, and quantify their morphologies using the fractional bulge luminosity (B/T). Within a single WFPC2 image, covering a field of ~3' (1 h Mpc at z = 0.25) in the cluster center, we find that the low-LX clusters are dominated by galaxies with low B/T (~0), while the high-LX clusters are dominated by galaxies with intermediate B/T (~0.4). We test whether this difference could arise from a universal morphology-density relation due to differences in the typical galaxy densities in the two samples. We find that small differences in the B/T distributions of the two samples persist with marginal statistical significance (98% confidence based on a binned χ2 test) even when we restrict the comparison to galaxies in environments with similar projected local galaxy densities. A related difference (also of low statistical significance) is seen between the bulge-luminosity functions of the two cluster samples, while no difference is seen between the disk luminosity functions. From the correlations between these quantities, we argue that the global environment affects the population of bulges, over and above trends seen with local density. On the basis of this result, we conclude that the destruction of disks through ram pressure stripping or harassment is not solely responsible for the morphology-density relation and that bulge formation is less efficient in low-mass clusters, perhaps reflecting a less rich merger history.