THE DEMOGRAPHY OF MASSIVE DARK OBJECTS IN GALAXY CENTERS

We construct dynamical models for a sample of 36 nearby galaxies with Hubble Space Telescope (HST) photometry and ground-based kinematics. T he models assume that each galaxy is axisymmetric, with a two-integral distribution function, arbitrary inclination angle, a position-indepe ndent stellar mass-to-light ratio Y, and a central massive dark object (MDO) of arbitrary mass M-.. They provide acceptable fits to 32 of th e galaxies for some value of M-. and Y; the four galaxies that cannot be fitted have kinematically decoupled cores. The mass-to-light ratios inferred for the 32 well-fitted galaxies are consistent with the fund amental-plane correlation Y infinity L-0.2, where L is galaxy luminosi ty. In all but six galaxies the models require at the 95% confidence l evel an MDO of mass M-. similar to 0.006M(bulge) =0.006YL. Five of the six galaxies consistent with M-. = 0 are also consistent with this co rrelation. The other (NGC 7332) has a much stronger upper limit on M-. . We predict the second-moment profiles that should be observed at HST resolution for the 32 galaxies that our models describe well. We cons ider various parameterizations for the probability distribution descri bing the correlation of the masses of these MDOs with other galaxy pro perties. One of the best models can be summarized thus: a fraction f s imilar or equal to 0.97 of early-type galaxies have MDOs, whose masses are well described by a Gaussian distribution in log (M-./M-bulge) of mean -2.28 and standard deviation similar to 0.51. There is also marg inal evidence that M-. is distributed differently for ''core'' and ''p ower law'' galaxies, with core galaxies having a somewhat steeper depe ndence on M-bulge.