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Title

Uniting old stellar systems: from globular clusters to giant ellipticals

Author(s)

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

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 ~10 4 M⊙ systems (GCs) to ~10 12 M⊙ systems (elliptical galaxies). The lack of measured velocity dispersions for dwarf ellipticals with -16 > MK > -18 (~10 8 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 10 12 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.

Publication type

Journal article

Research centre

Swinburne University of Technology. Faculty of Information and Communication Technologies. Centre for Astrophysics and Supercomputing

Source

Monthly Notices of the Royal Astronomical Society, Vol. 389, no. 4 (Oct 2008), pp. 1924-1936

Publication year

2008

Keyword(s)

2MASS;  Dwarf galaxies;  Elliptical and lenticular galaxies;  Fundamental parameters of galaxies;  Galaxy kinematics and dynamics;  GCs;  Globular clusters;  Two Micron All Sky Survey

Publisher

Wiley-Blackwell Publishing

ISSN

0035-8711

Publisher URL

http://dx.doi.org/10.1111/j.1365-2966.2008.13739.x

Copyright

Copyright © 2008 Royal Astronomical Society. Author's final draft reproduced in accordance with the copyright policy of the publisher.

Full text

Peer reviewed