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To better understand the mechanism or mechanisms that lead to AGN activity today, we measure the X-ray AGN fraction in a new sample of nearby clusters and examine how it varies with galaxy properties, projected clustercentric radius, and cluster velocity dispersion. We present new wide-field Chandra X-Ray Observatory observations of Abell 85, Abell 754, and the background cluster Abell 89B out to their virial radii. Out of 17 X-ray sources associated with galaxies in these clusters, we classify 7 as X-ray AGNs with 10^{41}"/> LX ,B > 1041 ergs s[?]1. Only 2 of these would be classified as AGNs based on their optical spectra. We combine these observations with archival data to create a sample of X-ray AGN from 6 z < 0.08 clusters and find that 3.4+ 1.1[?]0.8 % of MR < [?] 20 galaxies host X-ray AGNs with 10^{41}"/> LX ,B > 1041 ergs s[?]1. We find that more X-ray AGNs are detected in more luminous galaxies and attribute this to larger spheroids in more luminous galaxies and increased sensitivity to lower Eddington-rate accretion from black holes in those spheroids. At a given X-ray luminosity limit, more massive black holes can be accreting less efficiently, yet still be detected. If interactions between galaxies are the principal drivers of AGN activity, then the AGN fraction should be higher in lower velocity dispersion clusters and the outskirts of clusters. However, the tendency of the most massive and early-type galaxies to lie in the centers of the richest clusters could dilute such trends. While we find no variation in the AGN fraction with projected clustercentric radius, we do find that the AGN fraction increases significantly, from 2.6+ 1.0[?]0.8 % in rich clusters to 10.0+ 6.2[?]4.3 % in those with lower velocity dispersions.

We present a detailed (U - V) color-magnitude relation (CMR) analysis for three local (z < 0.06) clusters of galaxies. From square-degree imaging of the Abell clusters A85, A496, and A754, we select 637 galaxies down to L ~ 0.1Limg1.gif with spectroscopic membership, which minimizes uncertain field contamination corrections. To characterize the degree of CMR uniformity among nearby clusters, we use a maximum likelihood technique to quantify the CMR properties of the red galaxies in each cluster. We find that these clusters have similar CMRs with a mean color of (U - V) = 1.40 at MV = -20 + 5img2.gif h and narrow limits of intrinsic color scatter [0.047, 0.079] and slope [-0.094, -0.075]. If we restrict our analysis to the core cluster population of red galaxies, the resulting CMRs are in close agreement with that of the Coma Cluster, the only local cluster with a rest-frame (U - V) CMR determination of comparable precision. Therefore, the CMR uniformity of present-day clusters spans a fairly wide range of cluster masses. We test how sensitive the CMR uniformity is to variations in color aperture size and sample selection, and we find at most slightly wider limits of scatter [0.047, 0.112] and slope [-0.104, -0.054]. This upper limit for (U - V) scatter is consistent with the bulk of the stellar populations of red cluster galaxies forming before z = 1.2, with a maximum age spread of 5.2 Gyr. In addition, we note that using colors from apertures containing equal fractions of galactic light does not remove the CMR slope and that none of the slopes exhibit a break as claimed by Metcalf and coworkers. Our findings expand the single Coma data point and provide a much needed z = 0 baseline for comparisons to high-redshift cluster CMRs at the same rest-frame wavelengths. The range in CMR scatter that we find among nearby clusters is consistent with the values reported for clusters at higher redshifts, further suggesting that there has been little evolution in the stellar populations of red-sequence cluster galaxies out to at least z ~ 1. To identify the most recently accreted cluster galaxies, we divide each cluster's membership into three galaxy populations based on (U - V) color relative to the well-defined CMR. Blue and moderately blue galaxies make up 18%-23% by number of each cluster population more luminous than 0.1L*. Our color-magnitude division should represent a rough time since cluster accretion. In testing this hypothesis, we find that blue galaxies are spatially, kinematically, and morphologically distinct from red cluster galaxies. Even in projection, the blue galaxies reside toward the outskirts of the cluster and appear to avoid the inner half-megaparsec, in contrast with the increasing density of red-sequence galaxies toward the cluster center. In addition, the blue galaxies have velocity distributions relative to the cluster rest frame that are flatter, and some have different means, compared to the roughly Gaussian distribution of red member velocities. Members with the bluest colors tend to be disklike or irregular in appearance compared to the red galaxies, which have mostly early-type (E/S0) morphologies. Moderately blue cluster galaxies may be an intermediate mix with a fraction of small bulge-to-disk ratio S0s, yet these require closer scrutiny. The spatial, kinematic, and morphological distinctions between blue and red cluster galaxies provide further evidence that CMR-relative color is related to time since cluster infall and that bluer members are indeed the most recently accreted field spirals as expected in a hierarchical universe.

E+A galaxies, whose spectra have deep Balmer absorption lines but no significant [O II] emission, are the best candidates for an evolutionary link between star-forming, gas-rich galaxies and quiescent, gas-poor galaxies. However, their current morphologies are not well known. We present Hubble Space Telescope WFPC2 observations of the five bluest E+A galaxies (z ~ 0.1) in the sample of Zabludoff and coworkers to study whether their detailed morphologies are consistent with late- to early-type evolution and to determine what drives that evolution. The morphologies of four galaxies are disturbed, indicating that a galaxy-galaxy merger is at least one mechanism that leads to the E+A phase. Two-dimensional image fitting shows that the E+As are generally bulge-dominated systems, even though at least two E+As may have underlying disks. In the fundamental plane, E+As stand apart from the E/S0 galaxies mainly because of their high effective surface brightness. Fading of the young stellar population and the corresponding increase in their effective radii will cause these galaxies to migrate toward the locus of E/S0 galaxies. E+As have profiles qualitatively like those of normal power-law early-type galaxies but have higher surface brightnesses. This result provides the first direct evidence supporting the hypothesis that power-law elliptical galaxies form via gas-rich mergers. In total, at least four E+As are morphologically consistent with early-type galaxies. We detect compact sources, possibly young star clusters, associated with the galaxies. These sources are much brighter (MR ~ -13) than Galactic globular clusters, have luminosities consistent with the brightest clusters in nearby starburst galaxies, and have blue colors consistent with the ages estimated from the E+A galaxy spectra (several 108 yr). Further study of such young star cluster candidates might provide the elusive chronometer needed to break the age/burst-strength degeneracy for these postmerger galaxies.