The Acs Virgo Cluster Survey Xv. the Formation Efficiencies of Globular Clusters in Early-type Galaxies: the Effects of Mass and Environment

The fraction of stellar mass contained in globular clusters (GCs), also measured by number as the specific frequency, is a fundamental quantity that reflects both a galaxy’s early star formation and its entire merging history. We present specific frequencies, luminosities, and mass fractions for the globular cluster systems of 100 early-type galaxies in the ACS Virgo Cluster Survey. This catalog represents the largest homogeneous catalog of GC number and mass fractions across a wide range of galaxy luminosity (−22 < MB < −15). We find that 1) GC mass fractions can be high in both giants and dwarfs, but are universally low in galaxies with intermediate luminosities (−20 < MB < −17). 2) The fraction of red GCs increases with galaxy luminosity, but stays constant or decreases for galaxies brighter than Mz = −22. As a result, although specific frequencies for blue and red GCs are both higher in massive galaxies, the behavior of specific frequency across galaxy mass is dominated by the blue GCs. 3) The GC fractions of low-mass galaxies exhibit a dependence on environment, where dwarf galaxies closer to the cluster center have higher GC fractions. Nearly all dwarfs with high GC fractions are within 1 Mpc of the cD galaxy M87, presenting the first strong evidence that GC formation in dwarf galaxies is biased toward dense environments. 4) GC formation in central dwarfs is biased because their stars form earliest and most intensely. Comparisons to early-type dwarf galaxies in the Millennium Simulation show that central dwarfs are likely to have older stellar populations and form more of their stars at higher star formation rates (SFRs) and star formation rate surface densities. In addition, the SFR surface density in simulated dwarfs peaks before the total SFR, naturally producing GC populations that are older and more metal-poor than the field stars. 5) Dwarfs within ∼ 40 kpc of the giant ellipticals M87 and M49 are red for their luminosities and have few or no GCs, suggesting that they have been tidally stripped and have contributed their GCs to the halos of their giant neighbors. The central dwarfs with high GC mass fractions are thus likely to be the survivors most similar to the protogalaxies that assembled the rich M87 globular cluster system. Subject headings: galaxies: elliptical and lenticular, cD — galaxies: dwarf — galaxies: halos — galaxies: evolution — galaxies: star clusters – globular clusters: general 1 Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. 2 Herzberg Institute of Astrophysics, National Research Council of Canada, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada; [email protected] 3 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218, USA 4 Department of Astronomy, Peking University, Beijing 100871, China 5 Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138 6 Clay Fellow 7 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany 8 Departamento de Astronomı́a y Astrof́ısica, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile 9 Department of Physics & Astronomy, University of Hawai’i, 200 W. Kawili Street, Hilo, HI 96720, USA 10 European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile 11 Gemini Observatory, Casilla 603, La Serena, Chile 12 Department of Physics and Astronomy, Washington State University, Pullman, WA 99164-2814 13 Institute for Astronomy, National Central University Taiwan, Chung-Li 32054, Taiwan 14 University of Paris 7 Denis Diderot, 75205 Paris Cedex 13, France 15 GEPI, Observatoire de Paris, Section de Meudon, 5 Place J. Janssen, 92195 Meudon Cedex, France