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Title:
Globular Cluster Systems in Brightest Cluster Galaxies: Bimodal Metallicity Distributions and the Nature of the High-Luminosity Clusters
Authors:
Harris, William E.; Whitmore, Bradley C.; Karakla, Diane; Okoń, Waldemar; Baum, William A.; Hanes, David A.; Kavelaars, J. J.
Affiliation:
AA(Department of Physics and Astronomy, McMaster University, Hamilton, ON L8S 4M1; ), AB(Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218; ), AC(Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218; ), AD(Department of Physics and Astronomy, McMaster University, Hamilton, ON L8S 4M1; ), AE(Astronomy Department, University of Washington, Seattle, WA 98195; ), AF(Department of Physics, Queen's University, Kingston, ON K7L 3N6; ), AG(Herzberg Institute of Astrophysics, National Research Council of Canada, 5071 West Saanich Road, Victoria, BC V9E 2E7; )
Publication:
The Astrophysical Journal, Volume 636, Issue 1, pp. 90-114. (ApJ Homepage)
Publication Date:
01/2006
Origin:
UCP
ApJ Keywords:
Galaxies: Elliptical and Lenticular, cD, Galaxies: Star Clusters, Galaxy: Globular Clusters: General
DOI:
10.1086/498058
Bibliographic Code:
2006ApJ...636...90H

Abstract

We present new (B, I) photometry for the globular cluster systems in eight brightest cluster galaxies (BCGs), obtained with the ACS/WFC camera on the Hubble Space Telescope. In the very rich cluster systems that reside within these giant galaxies, we find that all have strongly bimodal color distributions that are clearly resolved by the metallicity-sensitive (B-I) index. Furthermore, the mean colors and internal color range of the blue subpopulation are remarkably similar from one galaxy to the next, to well within the +/-0.02-0.03 mag uncertainties in the foreground reddenings and photometric zero points. By contrast, the mean color and internal color range for the red subpopulation differ from one galaxy to the next by twice as much as the blue population. All the BCGs show population gradients, with much higher relative numbers of red clusters within 5 kpc of their centers, consistent with their having formed at later times than the blue, metal-poor population. A striking new feature of the color distributions emerging from our data is that for the brightest clusters (MI<-10.5) the color distribution becomes broad and less obviously bimodal. This effect was first noticed by Ostrov et al. and Dirsch et al. for the Fornax giant NGC 1399; our data suggest that it may be a characteristic of many BCGs and perhaps other large galaxies. Our data indicate that the blue (metal-poor) clusters brighter than MI~=-10 become progressively redder with increasing luminosity, following a mass/metallicity scaling relation Z~M0.55. A basically similar relation has been found for M87 by Strader et al. (2005). We argue that these GCS characteristics are consistent with a hierarchical-merging galaxy formation picture in which the metal-poor clusters formed in protogalactic clouds or dense starburst complexes with gas masses in the range 107-1010 Msolar, but where the more massive clusters on average formed in bigger clouds with deeper potential wells where more preenrichment could occur.
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