Uniting old stellar systems

from globular clusters to giant ellipticals

Duncan A. Forbes*, Paul Lasky, Alister W. Graham, Lee Spitler

*Corresponding author for this work

Research output: Contribution to journalArticle

91 Citations (Scopus)

Abstract

Elliptical galaxies and globular clusters (GCs) have traditionally been regarded as physically distinct entities due to their discontinuous distribution in key scaling diagrams involving size, luminosity and velocity dispersion. Recently this distinctness has been challenged by the discovery of stellar systems with mass intermediate between those of GCs and dwarf ellipticals (such as ultracompact dwarfs and dwarf galaxy transition objects). Here we examine the relationship between the virial and stellar mass for a range of old stellar systems, from GCs to giant ellipticals, and including such intermediate-mass objects (IMOs). Improvements on previous work in this area include the use of (i) near-infrared magnitudes from the Two Micron All Sky Survey (2MASS), (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 M systems (GCs) to ∼1012 M systems (elliptical galaxies). The lack of measured velocity dispersions for dwarf ellipticals with -16 > MK > -18 (∼108 M) 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 elliptical galaxies to have roughly equal fractions of dark and stellar matter within a virial radius; only in the most massive (greater than 1012 M) ellipticals 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 initial mass function (and hence their stellar mass-to-light ratios) or structural properties. We argue that most of these IMOs have similar properties to massive GCs, i.e. IMOs are essentially massive star clusters. Only the dwarf spheroidal galaxies exhibit behaviour notably distinct from the other stellar systems examined here, i.e. they display a strongly increasing virial-to-stellar mass ratio (equivalent to higher dark matter fractions) with decreasing stellar mass. The data used in this study are available in electronic format.

Original languageEnglish
Pages (from-to)1924-1936
Number of pages13
JournalMonthly Notices of the Royal Astronomical Society
Volume389
Issue number4
DOIs
Publication statusPublished - 2008
Externally publishedYes

Bibliographical note

Erratum can be found in Monthly Notices of the Royal Astronomical Society, Volume 415(3), 2976, http://dx.doi.org/10.1111/j.1365-2966.2011.18713.x

Keywords

  • galaxies: dwarf
  • galaxies: elliptical and lenticular, cD
  • galaxies: fundamental parameters
  • galaxies: general
  • galaxies: kinematics and dynamics
  • globular clusters: general

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