Low-mass X-ray binaries indicate a top-heavy stellar initial mass
  function in ultra compact dwarf galaxies
  It has been shown before that the high mass-to-light ratios of ultra compact
dwarf galaxies (UCDs) can be explained if their stellar initial mass function
(IMF) was top-heavy, i.e. that the IMF was skewed towards high mass stars. In
this case, neutron stars and black holes would provide unseen mass in the UCDs.
In order to test this scenario with an independent method, we use data on which
fraction of UCDs has a bright X-ray source. These X-ray sources are interpreted
as low-mass X-ray binaries (LMXBs), i.e. binaries where a neutron star accretes
matter from an evolving low-mass star. We find that LMXBs are indeed up to 10
times more frequent in UCDs than expected if the IMF was invariant. The
top-heavy IMF required to account for this overabundance is the same as needed
to explain the unusually high mass-to-light ratios of UCDs and a top-heavy IMF
appears to be the only simultaneous explanation for both findings. Furthermore,
we show that the high rate of type II supernovae (SNII) in the star-burst
galaxy Arp 220 suggests a top-heavy IMF in that system. This finding is
consistent with the notion that star-burst galaxies are sites where UCDs are
likely to be formed and that the IMF of UCDs is top-heavy. It is estimated that
the IMF becomes top-heavy whenever the star formation rate per volume surpasses
0.1 Solar masses per year and cubic parsec in pc-scale regions.
