Coma Cluster Treasury Survey

Project Overview - Objective 6

Nuclear Star Clusters and Central Massive Black Holes

The masses of black holes at the centers of spheroidal stellar systems correlate with the large-scale properties of the host spheroid (i.e. elliptical galaxy or the bulge of a disk galaxy). Popular correlations involve the spheroid's concentration of stars, its total luminosity, mass and internal dynamics (velocity dispersion of the stars).

The relationship between the nuclear star clusters which reside at the centers of dwarf galaxies, and the massive black holes which are found in giant galaxies, is of increasing interest - although the formation of both remains poorly understood. Nuclear star clusters are observed in about 80% of intermediate-luminosity, early-type galaxies and their luminosity in dwarf elliptical galaxies is known to correlate strongly with the luminosity of the host galaxy. This luminosity trend has also been shown to exist in the bulges of lenticular and early-type spiral galaxies. Moreover, it has been proposed that nuclear star clusters may follow some of the same mass relations that are defined by supermassive black holes.

It has been suggested that the primary physical connection may be with the central stellar density (prior to core depletion, see below) rather than a global property of the host spheroid. Given the known trend between central stellar density and host spheroid luminosity, the known relations between nuclear cluster and black hole mass with global properties of the spheroid may only be secondary in nature. The Coma Cluster Treasury Survey will enable us to explore this issue by examining the connection between the mass of the nuclear star clusters and the central stellar density of the host spheroid.

Giant elliptical galaxies display cores that are partially depleted of stars. A possible mechanism is the slingshot action of supermassive black holes - from progenitor galaxies - as they sink to the center of the newly formed galaxy during a merger event. Cores may also be enlarged when black holes are ejected from galaxy centers by the gravitational wave rocket effect following coalescence of a massive black hole binary. From our HST images we will be able to quantify the sizes (and mass deficits, Mdef) of these cores, and also to predict each galaxy's central black hole mass (Mbh) from the above mentioned correlations. Given that the mass ratio Mdef/Mbh varies linearly with N, where N is the number of major dry mergers, such measurements can be used to place constraints on the dry merger history of such cluster galaxies and to constrain N as a function of galaxy mass. Such quantitative information reveals how galaxies grow within the cluster environment.

Measurement of spatial offsets between nuclear clusters and the outer isophotes of their host galaxy may reveal an oscillation of the nucleus about the center of the gravitational potential, which will be of larger amplitude in shallower potentials. Furthermore, apparent double nuclei may in some cases be a sign of an edge-on nuclear disk from which we may be able to determine the presence of a central massive black hole. Finally, the radial distribution of nucleated galaxies within the cluster, and comparison with the lower density Fornax and Virgo galaxy clusters, may also help shed some light on potential formation mechanisms, as may studies of the relation between the nuclear and host galaxy colour.