SCYON Abstract

Received on September 6 2006

The ACS Virgo Cluster Survey. XIV. Analysis of Color-Magnitude Relations in Globular Cluster Systems

AuthorsSteffen Mieske (1), Andrés Jordán (1,2), Patrick Côté (3), Markus Kissler-Patig (1), Eric W. Peng (3), Laura Ferrarese (3), John P. Blakeslee (4,5), Simona Mei (4), David Merritt (6), John L. Tonry (7) and Michael J. West (8)
(1) European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
(2) Astrophysics, Denys Wilkinson Building, University of Oxford, 1 Keble Road, OX1 3RH, UK
(3) Herzberg Institute of Astrophysics, Victoria, BC V9E 2E7, Canada
(4) Department of Physics and Astronomy, The Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218-2686
(5) Department of Physics and Astronomy, PO Box 642814, Washington State University, Pullman, WA 99164
(6) Department of Physics, Rochester Institute of Technology, 84 Lomb Memorial Drive, Rochester, NY 14623
(7) Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822
(8) Gemini Observatory Southern Operations Center c/o AURA, Casilla 603 La Serena, Chile
Accepted byAstrophysical Journal


We examine the correlation between globular cluster (GC) color and magnitude using HST/ACS imaging for a sample of 79 early-type galaxies (-21.7 < MB < -15.2 mag) with accurate SBF distances from the ACS Virgo Cluster Survey. Using the KMM mixture modeling algorithm, we find a highly significant correlation, d(g-z)/dz = -0.037 ± 0.004, between color and magnitude for the subpopulation of blue GCs in the co-added GC color-magnitude diagram of the three brightest Virgo galaxies (M49, M87 and M60): brighter GCs are redder than their fainter counterparts. For the single GC systems of M87 and M60, we find similar correlations; M49 does not appear to show a significant trend. There is no correlation between (g-z) and Mz for GCs of the red subpopulation. The correlation d(g-z)/dg for the blue subpopulation is much weaker than d(g-z)/dz. Using Monte Carlo simulations, we attribute this to the fact that the blue subpopulation in Mg extends to higher luminosities than the red subpopulation, which biases the KMM fits. The correlation between color and Mz thus is a real effect. This conclusion is supported by biweight fits to the same color distributions. We identify two environmental dependencies of the color-magnitude relation: (1) the slope decreases in significance with decreasing galaxy luminosity; and (2) the slope is stronger for GCs at smaller galactocentric distances. We examine several mechanisms that might give rise to the observed color-magnitude relation: (1) presence of contaminators; (2) accretion of GCs from low-mass galaxies; (3) stochastic effects; (4) capture of field stars by individual GCs; and (5) GC self-enrichment. We conclude that self-enrichment and field-star capture, or a combination of these processes, offer the most promising means of explaining our observations.