The Impact of Stripped Nuclei on the Supermassive Black Hole Number Density in the Local Universe

The recent discovery of supermassive black holes (SMBHs) in high mass ultra-compact dwarf galaxies (UCDs) suggests that at least some UCDs are the nuclear star clusters of stripped galaxies. In this paper we present a new method to estimate how many UCDs host an SMBH and thus are stripped galaxy nuclei. We revisit the dynamical mass measurements that suggest many UCDs have more mass than expected from stellar population estimates, which observations have shown is due to the presence of an SMBH. We revise the stellar population mass estimates using a new empirical relation between the mass-to-light ratio (M/L) and metallicity to predict which UCDs most likely host an SMBH. We calculate the fraction of UCDs that host SMBHs across their entire luminosity range for the first time. We then apply the SMBH occupation fraction to the observed luminosity function of UCDs and estimate that in the Fornax and Virgo clusters alone there should be {69}_-25⁺³² stripped nuclei with SMBHs. This analysis shows that stripped nuclei are almost as common in clusters as present-day galaxy nuclei. We estimate the SMBH number density caused by stripped nuclei to be (2-8) × 10^-3 Mpc^-3, which represents a significant fraction (8%-32%) of the SMBH density in the local universe. These SMBHs hidden in stripped nuclei increase expected event rates for tidal disruption events and SMBH-SMBH and SMBH-BH mergers. The existence of numerous stripped nuclei with SMBHs are a direct consequence of hierarchical galaxy formation, but until now their impact on the SMBH density had not been quantified.