Uniting Old Stellar Systems: From Globular Clusters to Giant Ellipticals

Authors: Duncan Forbes, Paul Lasky, Alister W. Graham, Lee Spitler

Abstract: We examine the relationship between the virial and stellar mass for a range of old stellar systems, from GCs to giant ellipticals, and Intermediate Mass Objects (IMOs). Improvements on previous work in this area include the use of (i) near-infrared magnitudes from the 2MASS survey, (ii) aperture corrections to velocity dispersions, (iii) homogeneous half light radii and (iv) accounting for the effects of non-homology in galaxies. We find a virial-to-stellar mass relation that ranges from ~104 MSun systems (GCs) to ~1011 MSun systems (Elliptical galaxies). The lack of measured velocity dispersions for dwarf ellipticals with -16 > MK > -18 (~108 MSun) currently inhibits our ability to determine how, or indeed if, these galaxies connect continuously with GCs in terms of their virial-to-stellar mass ratios. We find that elliptical galaxies have roughly equal fractions of dark and stellar matter; only in the most massive (greater than 1011 MSun) Elliptical galaxies does dark matter dominate the virial mass. Although the IMOs reveal slightly higher virial-to-stellar mass ratios than lower mass GCs, this may simply reflect our limited understanding of their IMF (and hence their stellar mass-to-light ratios) or their structural properties. We argue that most of these intermediate mass objects are a natural extension of the GC sequence to higher masses, i.e. IMOs are essentially massive star clusters.