Top-heavy IMFs in ultra compact dwarf galaxies?

Abstract

Ultra compact dwarf galaxies (UCDs) are dense stellar systems at the border between massive star-clusters and small galaxies. Their on average high optical mass-to-light (M/L) ratio cannot be explained by stellar populations with the canonical stellar initial mass function (IMF), while it is doubtful that non-baryonic dark matter can accumulate enough on the scales of UCDs for influencing their dynamics significantly. UCDs in the Virgo galaxy cluster apparently also have an over-abundance of neutron stars, strongly suggesting a top-heavy IMF, which would explain both findings. This is because a top-heavy IMF can provide the unseen mass through an abundance of stellar remnants. The suggested variation of the IMF can be understood if UCDs represent a case of rapid star-formation in an extremely dense environment. While top-heavy IMFs imply a much heavier mass-loss shortly after the formation of a stellar system, this process does not necessarily dissolve the UCDs. Their formation with a top-heavy IMF would therefore not contradict their existence.