Mass loss and expansion of ultra compact dwarf galaxies through gas expulsion and stellar evolution for top-heavy stellar initial mass functions

The dynamical V-band mass-to-light ratios of ultra compact dwarf galaxies (UCDs) are higher than predicted by simple stellar population models with the canonical stellar initial mass function (IMF). One way to explain this finding is a top-heavy IMF so that the unseen mass is provided by additional remnants of high-mass stars. A possible explanation for why the IMF in UCDs could be top-heavy while this is not the case in less massive stellar systems is that encounters between protostars and stars become probable in forming massive systems. However, the required number of additional stellar remnants proves to be rather high, which raises the question of how their progenitors would affect the early evolution of a UCD. We have therefore calculated the first 200 Myr of the evolution of the UCDs, using the particle-mesh code superbox. It is assumed that the stellar populations of UCDs were created in an initial starburst, which implies heavy mass loss during the following ≈40 Myr due to primordial gas expulsion and supernova explosions. This mass loss is modelled by reducing the mass of the particles according to tabulated mass-loss histories which account for different IMFs, star formation efficiencies (SFEs), heating efficiencies (HEs), initial masses and initial extensions of the computed UCDs. For each combination of SFE and HE, we find objects that roughly resemble UCDs at the end of the simulation. For low SFEs, the IMF would have to be steeper than in the case of very high SFEs for the models not to expand too much. However, the main conclusion is that the existence of UCDs does not contradict the notion that their stellar populations formed rapidly and with a top-heavy IMF. We find tentative evidence that the UCDs may have had densities as high as 108 M⊙ pc−3 at birth. This will have to be confirmed by follow-up modelling.