The Demography of Massive Dark Objects in Galaxy Centers

We construct dynamical models for a sample of 36 nearby galaxies with HubbleSpace Telescope photometry and ground-based kinematics. The models assume thateach galaxy is axisymmetric, with a two-integral distribution function,arbitrary inclination angle, a position-independent stellar mass-to-light ratioUpsilon, and a central massive dark object (MDO) of arbitrary mass M_bh. Theyprovide acceptable fits to 32 of the galaxies for some value of M_bh andUpsilon; the four galaxies that cannot be fit have kinematically decoupledcores. The mass-to-light ratios inferred for the 32 well-fit galaxies areconsistent with the fundamental plane correlation Upsilon \propto L^0.2, whereL is galaxy luminosity. In all but six galaxies the models require at the 95%confidence level an MDO of mass M_bh ~ 0.006 M_bulge = 0.006 Upsilon L. Five ofthe six galaxies consistent with M_bh=0 are also consistent with thiscorrelation. The other (NGC 7332) has a much stronger upper limit on M_bh. Weconsider various parameterizations for the probability distribution describingthe correlation of the masses of these MDOs with other galaxy properties. Oneof the best models can be summarized thus: a fraction f ~0.97 of galaxies haveMDOs, whose masses are well described by a Gaussian distribution in log(M_bh/M_bulge) of mean -2.27 and width ~0.07.Comment: 28 pages including 13 figures and 4 tables. Submitted to A