DYNAMICAL CONSTRAINTS ON ALTERNATIVES TO SUPERMASSIVE BLACK-HOLES IN GALACTIC NUCLEI

The compelling dynamical evidence for massive dark objects in galactic nuclei does not uniquely imply massive black holes (BHs). To argue co nvincingly that these objects are BHs we must rule out alternatives to a BH, and the alternative to a point mass is a cluster of some sort o f nonluminous objects, such as a cluster of brown dwarfs or stellar re mnants. We use simple physical considerations to derive the maximum po ssible lifetime of a dark cluster that may consist of any plausible fo rm of nonluminous gravitating objects-from brown dwarfs and very low m ass objects of cosmic composition to white dwarfs, neutron stars, and black holes. The lower this limit relative to the galaxy age, the more implausible the cluster hypothesis is, thus arguing for a point mass. A cluster with a lifetime much shorter than 10 Gyr is unacceptable, s ince observing it at the present epoch would be highly improbable. Sin ce the goal is to rule out a dark cluster by showing that its lifetime must be very short, Lye make the most generous assumptions possible u nder the observational constraints to allow for its survival. We find that the lifetime of such a hypothetical cluster must be much shorter than the galaxy age only in the cases of NGC 4258 and our Galaxy, thus strongly arguing for a point mass. In all other galaxies, the case of a massive BH, although compelling, is not yet watertight. We also not e that there are two exotic alternatives to a massive BH that cannot b e ruled out even in the cases of NGC 4258 and the Galaxy: clusters of elementary particles (e.g., bosons), and clusters of very low mass (le ss than or similar to 0.04 M-.) BHs. We point our, however, serious di fficulties with these alternatives, and argue that they are highly imp lausible.