2008MNRAS.386..864D

Objects with radii of 10-100pc and masses in the range from 10⁶ to 10⁸M_solar have been discovered during the past decade. These so-called ultracompact 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 10⁶M_solar can be confirmed. There is a continuous transition to the typical, mass-independent 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, t_rel, which becomes longer than a Hubble time, τ_H, in the mass interval between 10⁶ and 10⁷M_solar. 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-to-light ratios typical for GCs happens again at a mass of ∼10⁶M_solar. 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 us 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 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 from the canonical one for resolved populations. But it is noteworthy that evidence for dark matter emerges only in systems with t_rel≳ τ_H.