Invited review abstract

Ultra compact dwarf galaxies (UCDs) show a larger M/L than star clusters but are collisionless. The high M/L values may be a result of a top-heavy IMF contributing remnants in large numbers. Also, some globular clusters show evidence for an abnormally depleted low-mass stellar mass function. This may be due to significant mass loss from a top-heavy IMF in an initially mass segregated cluster. These scenarios allow us (with Joerg Dabringhausen and Michael Marks) to distill a possible systematically varying IMF top-heaviness with star-burst density.