SCYON Abstract

Received on December 14 2005

Bimodal Galaxies and Bimodality in Globular Cluster Systems

AuthorsDuncan A. Forbes
Swinburne University
Accepted byAstrophysical Journal


Various galaxy properties are not continuous over a large range in mass, but rather reveal a remarkable transition or "bimodality" at a stellar mass of 3x1010 Msun. These properties include colors, stellar populations, Xray emission and mass-to-light ratios. This behavior has been interpreted as the transition from hot to cold flows by Dekel & Birnboim (2005). Here we explore whether globular cluster (GC) systems also reveal a bimodal nature with regard to this critical mass scale. Globular clusters probe star formation at early epochs in the Universe and survive subsequent galaxy mergers and accretions. We use new data from the ACS Virgo Cluster Survey (Peng et al. 2005), which provides a homogeneous sample of the GC systems around one hundred Virgo early-type galaxies covering a range of five hundred in galaxy mass. Their classification of the GC color distributions is taken to examine a key quantity -- the number of GCs per unit galaxy luminosity. Below the critical mass, this quantity (called the GC specific frequency) increases dramatically in its mean value and spread. This increase may be due to regulated star formation in low mass galaxies, which in turn is due to mass loss via winds and the transition from hot to cold gas accretion flows. We also note that above the critical mass, galaxies possess two GC subpopulations (with blue and red mean colors) but below this mass, galaxies rev eal an increasing proportion of single (blue) GC systems.