Nicholas Scott, Alister W. Graham
We investigate whether nuclear star clusters and supermassive black holes follow a common set of mass scaling relations with their host galaxy's properties, and hence can be considered to form a single class of central massive object. We have compiled a large sample of galaxies with measured nuclear star cluster masses and host galaxy properties from the literature and fit log-linear scaling relations. We find that nuclear star cluster mass, M_{NC}, correlates most tightly with the host galaxy's velocity dispersion: log M_{NC} = (2.11 \pm 0.31) \log (\sigma/54) + (6.63 \pm 0.09), but has a slope dramatically shallower than the relation defined by supermassive black holes. We find that the nuclear star cluster relations involving luminosity, mass and dynamics, intercept with but are in general shallower than the corresponding black hole scaling relations. In particular M_{NC} \propto M_{Gal,dyn}^{0.55 \pm 0.15}, whereas M_{BH} \propto M_{Gal,dyn}^{1.37 \pm 0.23} for our sample of massive black holes, when fit with a single power-law. We also find that the nuclear cluster mass is not a constant fraction of its host galaxy's or spheroid's mass. We conclude that nuclear stellar clusters and supermassive black holes do not form a single family of central massive objects.
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http://arxiv.org/abs/1205.5338
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