2002ApJ...566..123B

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-L_X sample. The remaining nine clusters comprise a high-L_X subsample with L_X>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, σ_z/z∼0.15), and the data are homogeneous in terms of depth, resolution (0".17=1h^–1₅₀ 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+5logh₅₀ (roughly 0.01L^*_R) with parametric models, and quantify their morphologies using the fractional bulge luminosity (B/T). Within a single WFPC2 image, covering a field of ∼3' (1h^–1₅₀ Mpc at z=0.25) in the cluster center, we find that the low-L_X clusters are dominated by galaxies with low B/T (∼0), while the high-L_X 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.