Uniting Old Stellar Systems: From Globular Clusters to Giant Ellipticals

TL;DR

This study investigates the relationship between virial and stellar mass across a spectrum of old stellar systems, revealing a continuous sequence from globular clusters to giant ellipticals and highlighting the role of dark matter in these systems.

Contribution

The paper introduces improved methods for analyzing stellar systems and demonstrates a continuous virial-to-stellar mass relation from GCs to ellipticals, including the role of dark matter.

Findings

Elliptical galaxies have roughly equal dark and stellar matter within the virial radius.

Dark matter dominates in the most massive ellipticals (>10^11 M_o).

IMOs are likely massive star clusters extending the GC sequence.

Abstract

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 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 ~10$^4$ M_o systems (GCs) to ~10$^11$ M_o systems (elliptical galaxies). The lack of measured velocity dispersions for dwarf ellipticals with -16 > M_K > -18 (~10$^8$ M_o) 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$^11$ M_o) 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 IMF (and hence their stellar mass-to-light ratios) or 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. [Abridged].