Black hole and nuclear cluster scaling relations: M_bh ~ M_nc^(2.7+/-0.7)

There is a growing array of supermassive black hole and nuclear star cluster scaling relations with their host spheroid, including a bent (black hole mass)-(host spheroid mass) M_bh-M_sph relation and different (massive compact object mass)-(host spheroid velocity dispersion) M_mco-sigma relations for black holes and nuclear star clusters. By combining the observed M_bh ~ sigma^(5.5) relation with the observed M_nc ~ sigma^(1.6--2.7) relation, we derive the expression M_bh ~ (M_nc)^(2--3.4), which should hold until the nuclear star clusters are eventually destroyed in the larger core-Sersic spheroids. This new mass scaling relation helps better quantify the rapid evolutionary growth of massive black holes in dense star clusters, and the relation is consistently recovered when coupling the observed M_nc ~ (M_sph)^(0.6--1.0) relation with the recently observed quadratic relation M_bh ~ (M_sph)^2 for Sersic spheroids.