The Lost Dwarfs of Centaurus A and the Formation of its Dark Globular Clusters

Abstract: We present theoretical constraints for the formation of the newly discovered dark star clusters (DSCs) with high mass-to-light (M/L) ratios, from Taylor et al (2015). These compact stellar systems photometrically resemble globular clusters (GCs) but have dynamical M/L ratios of ~ 10 - 100, closer to the expectations for dwarf galaxies. The baryonic properties of the dark star clusters (DSCs) suggest their host dark matter halos likely virialized at high redshift with M > 10^8 M_sun. We use a new set of high-resolution N-body simulations of Centaurus A to determine if there is a set of z=0 subhalos whose properties are in line with these observations. While we find such a set of subhalos, when we extrapolate the dark matter density profiles into the inner 20 pc, no dark matter halo associated with Centaurus A in our simulations, at any redshift, can replicate the extremely high central mass densities of the DSCs. Among the most likely options for explaining 10^5 - 10^7 M_sun within 10 pc diameter subhalos is the presence of a central massive black hole. We, therefore, propose that the DSCs are remnant cusps of stellar systems surrounding the central black holes of dwarf galaxies which have been almost completely destroyed by interactions with Centaurus A.