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

TL;DR

This study models the early evolution of ultra compact dwarf galaxies with top-heavy IMFs, showing that rapid formation and gas expulsion can produce objects resembling observed UCDs, supporting the top-heavy IMF hypothesis.

Contribution

First simulation of UCD evolution over 200 Myr incorporating gas expulsion and stellar remnants, demonstrating formation plausibility with a top-heavy IMF.

Findings

UCDs can form rapidly with top-heavy IMFs.

Gas expulsion significantly influences early UCD evolution.

Possible initial densities up to 10^8 M_sun/pc^3.

Abstract

(abridged) 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 proto-stars 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 approximately 40 Myr due to primordial gas expulsion and supernova explosions. We find at the end of the simulations for various initial conditions and (tabulated) mass-loss histories objects that roughly resemble UCDs. Thus, 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 10^8 M_sun/pc^3 at birth.