A STRINGENT CONSTRAINT ON ALTERNATIVES TO A MASSIVE BLACK HOLE AT THE CENTER OF NGC 4258

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Abstract
There is now dynamical evidence for massive dark objects at the center of several galaxies, but suggestions that these are supermassive black holes are based only on indirect astrophysical arguments. The recent unprecedented measurement of the rotation curve of maser emission sources at the center of NGC 4258, and the remarkable discovery that it is Keplerian to high precision, provides us a unique opportunity for testing alternatives to a BH (e.g., a massive cluster of stellar remnants, brown dwarfs, low-mass stars, or halo dark matter). We use a conservative upper limit on the systematic deviation from a Keplerian rotation curve to constrain the mass distribution at the galaxy center. Based on evaporation and physical collision time-scale arguments, we show that a central cluster is ruled out, *unless* the cluster consists of *extremely* dense objects with mass less than about 0.05 solar masses (e.g., low mass BHs or elementary particles). Since both of these dynamically-allowed systems are very improbable for other astrophysical reasons, we conclude that a central dense cluster at the center of NGC 4258 is *very* improbable, thus leaving the alternative possibility of a massive BH. We also show that the mass of the BH must be at least 98% of the mass enclosed within the inner edge of the masering disk (3.6*10^7 solar masses). A substantial contribution to that mass from a density cusp in the background mass distribution is excluded.

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