Dynamical versus Stellar Masses of Ultracompact Dwarf Galaxies in the
  Fornax Cluster
  The origin of ultracompact dwarf (UCD) galaxies, compact extragalactic
stellar systems, is still a puzzle for present galaxy formation models. We
present the comprehensive analysis of high resolution multi-object
spectroscopic data for a sample of 24 Fornax cluster UCDs obtained with VLT
FLAMES. It comprises previously published data for 19 objects (Mieske et al.
2008) which we re-analysed, including 13 with available HST photometric data.
Using Virtual Observatory technologies we found archival HST images for two
more UCDs and then determined their structural properties. For all objects we
derived internal velocity dispersions, stellar population parameters, and
stellar mass-to-light ratios (M/L)* by fitting individual simple stellar
population (SSP) synthetic spectra convolved with a Gaussian against the
observed spectra using the NBursts full spectral fitting technique. For 14
objects we estimated dynamical masses suggesting no dark matter (DM) in 12 of
them and no more than 40 per cent DM mass fraction in the remaining two, in
contrast to findings for several UCDs in the Virgo cluster. Some Fornax UCDs
even have too high values of (M/L)* estimated using the Kroupa stellar initial
mass function (IMF) resulting in negative formally computed DM mass fractions.
The objects with too high (M/L)* ratios compared to the dynamical ones have
relatively short dynamical relaxation timescales, close to the Hubble time or
below. We therefore suggest that their lower dynamical ratios (M/L)dyn are
caused by low-mass star depletion due to dynamical evolution. Overall, the
observed UCD characteristics suggest at least two formation channels: tidal
threshing of nucleated dwarf galaxies for massive UCDs (~10^8 M_sun), and a
classical scenario of red globular cluster formation for lower-mass UCDs (<
10^7 M_sun).
