On the relation between the mass of Compact Massive Objects and their host galaxies

Supermassive black holes and/or very dense stellar clusters are found in the central regions of galaxies. Nuclear star clusters (NSCs) are present mainly in faint galaxies, while supermassive black holes are common in galaxies with masses ≥10¹⁰ M_⊙. In the intermediate galactic mass range, both types of compact massive objects (CMOs) are found. Here, we present our collection of a huge set of NSC and massive black hole data that enlarges significantly already existing data bases useful to investigate for correlations of their absolute magnitudes, velocity dispersions and masses with structural parameters of their host galaxies. In particular, we directed our attention to some differences between the correlations of NSCs and massive black holes as subsets of CMOs with hosting galaxies. In this context, the mass-velocity dispersion relation plays a relevant role because it seems the one that shows a clearer difference between the supermassive black holes and NSCs. The M_MBH-σ has a slope of 5.19 ± 0.28, while M_NSC-σ has the much smaller slope of 1.84 ± 0.64. The slopes of the CMO mass-host galaxy B magnitude of the two types of CMOs are indistinguishable within the errors, while that of the NSC mass-host galaxy mass relation is significantly smaller than for supermassive black holes. Another important result is the clear depauperation of the NSC population in bright galaxy hosts, which reflects also in a clear flattening of the NSC mass versus host galaxy mass at high host masses.