Testing the Universality of the (U-V) Color-Magnitude Relations for Nearby Clusters of Galaxies

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.