From star clusters to dwarf galaxies: the properties of dynamically hot stellar systems

By Joerg Dabringhausen, Michael Hilker, Pavel Kroupa. 2008

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Objects with radii of 10pc to 100pc and masses in the range from 10 6 M⊙ to 10 8 M⊙ have been discovered during the past decade. These so-called ultra compact dwarf galaxies (UCDs) constitute a transition between classical star clusters and elliptical galaxies in terms of radii, relaxation times and V -band mass-to-light ratios. Using new data, the increase of typical radii with mass for compact objects more massive than 10 6 M⊙ can be confirmed. There is a continuous transition to the typical, massindependent radii of globular clusters (GCs). It can be concluded from the different relations between mass and radius of GCs and UCDs that at least their evolution must have proceeded differently, while the continuous transition could indicate a common formation scenario. The strong increase of the characteristic radii also implies a strong increase of the median two-body relaxation time, trel, which becomes longer than a Hubble time, τH, in the mass interval between 10 6 M⊙ and 10 7 M⊙. This is also the mass interval where the highest stellar densities are reached. The mass-to-light ratios of UCDs are clearly higher than the ones of GCs, and the departure from mass-tolight ratios typical for GCs happens again at a mass of � 10 6 M⊙. Dwarf spheroidal galaxies turn out to be total outliers compared to all other dynamically hot stellar systems regarding their dynamical mass-to-light ratios. Stellar population models were consulted in order to compare the mass-to-light ratios of the UCDs with theoretical predictions for dynamically unevolved simple stellar populations (SSPs), which are probably a good approximation to the actual stellar populations in the UCDs. The SSP models also allow to account for the effects of metallicity on the mass-to-light ratio. It is found that the UCDs, if taken as a sample, have a tendency to higher mass-to-light ratios than it would be expected from the SSP models assuming that the initial stellar mass function in the UCDs is the same as in resolved stellar populations. This can be interpreted in several ways: As a failure of state-of-the-art stellar evolution and stellar population modelling, as a presence of dark matter in UCDs or as stellar populations which formed with initial stellar mass functions different to the canonical one for resolved populations. But it is noteworthy that evidence for dark matter emerges only in systems with trel ? τH.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/03198dc00b9ee58cda66dc39dd34255d8da5ad5f

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Citations in the corpus

The local nanohertz gravitational-wave landscape from supermassive black hole binaries (2017, unknown): “4 Ref (Dabringhausen et al. 2008), Figure 7.”
Local-Group tests of dark-matter concordance cosmology - Towards a new paradigm for structure formation (2010, methodology): “Upper panel: The dynamical (M/L)dyn ratio (calculated assuming Newtonian dynamics to be valid in the weakfield limit) in dependence of the luminosity,LV , for pressuresupported stellar systems following Dabringhausen et al. (2008).”
The Orbital Poles of Milky Way Satellite Galaxies: A Rotationally Supported Disk of Satellites (2008, background): “…in simulations of CDM-dominated halos are in disagreement with the density profiles inferred for MW dSph satellites (Gilmore et al. 2007), but these are believed to be the most dark matterYdominated objects currently known (Wilkinson et al. 2002; Kleyna et al. 2003; Dabringhausen et al. 2008).”
Accurate masses for dispersion-supported galaxies (2009, background): “Nevertheless, it seems that dark matter plays a clearly dominant dynamical role (ΥI 1/2 & 5) within r 1/2 in only the most extreme systems (see similar results by Dabringhausen et al. 2008; Forbes et al. 2008, who study slightly more limited ranges of spheroidal galaxy luminosities).”
Families of dynamically hot stellar systems over 10 orders of magnitude in mass (2011, background): “It is, however, interesting to note that these low mass dwarf galaxies are the only objects which exhibit very high dynamical massto-light ratios (e.g. Dabringhausen et al. 2008; Forbes et al. 2008).”
The relation between the most-massive star and its parental star cluster mass (2009, background): “A log-normal form below 1 M⊙ with a power-law extension to high masses was suggested by Chabrier (2003) but is indistinguishable from the canonical form (Dabringhausen et al. 2008) and does not cater for the discontinuity.”
Inclination- and dust-corrected galaxy parameters: bulge-to-disc ratios and size–luminosity relations (2008, background): “From −18 < MB < −13 mag, the effective radii of dwarf elliptical galaxies display no real trend with magnitude but instead scatter around a value of ∼1 kpc (Smith Castelli et al. 2008; Dabringhausen et al. 2008; Figure 11a).”
A MegaCam Survey of Outer Halo Satellites. III. Photometric and Structural Parameters (2018, background): “…structural properties of stellar systems in the Milky Way, M31 and nearby galaxy groups and clusters (e.g., Kormendy 1990; Côté et al. 2002; Willman et al. 2005a; Belokurov et al. 2007a; Dabringhausen et al. 2008; Tollerud et al. 2011; Huxor et al. 2011; Misgeld & Hilker 2011; Norris et al. 2014).”
THE NEXT GENERATION VIRGO CLUSTER SURVEY (NGVS). I. INTRODUCTION TO THE SURVEY (2012, background): “…GCs and UCDs appear to show a “break” at ≈2 × 106 solar masses (Haşegan et al. 2005; Mieske et al. 2008), suggesting that UCDs may represent a distinct population of compact stellar systems that are intermediate in properties between GCs and cEs (Haşegan et al. 2005; Dabringhausen et al. 2008).”
Quantifying the coexistence of massive black holes and dense nuclear star clusters (2009, background): “The absolute V -band magnitude from the updated Dynamical-to-stellar mass ratios of 2 to 3 are not uncommon for bright globular clusters (e.g., Dabringhausen et al. 2008; Forbes et al. 2008).”
Evidence for top-heavy stellar initial mass functions with increasing density and decreasing metallicity (2012, background): “Large mass-to-light ratios in UCDs indicate this class of objects to have formed with a top-heavy IMF as well (Dabringhausen et al. 2008).”
The Dark Matter Crisis: Falsification of the Current Standard Model of Cosmology (2012, unknown): “3 109M}) that b1¼ 0.593 0.027, b0¼ 2.99 0.30, as already shown by Dabringhausen et al. (2008).”
THE RELATIONSHIPS AMONG COMPACT STELLAR SYSTEMS: A FRESH VIEW OF ULTRACOMPACT DWARFS (2011, background): “…studies have sought to understand where these curious objects fit in the multi-dimensional parameter space for hot (dispersionsupported) stellar systems that encompass massive and dwarf galaxies as well as compact and extended star clusters (Dabringhausen et al. 2008; Forbes et al. 2008).”
Multiple populations in globular clusters (2012, background): “(2007): the globular clusters must have been initially a large factor (10-100) more massive than today and almost all the first generation stars must have been preferentially lost.”