A Stringent Constraint on Alternatives to a Massive Black Hole at the Center of NGC 4258

Open Access
Publisher Website
Google Scholar
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. As emphasized by Kormendy and Richstone, large M/L ratios and gas motions of order ≈103 km s-1 do not uniquely imply a massive black hole (BH), and it is possible that the central dark objects in these galaxies are massive clusters of stellar remnants, brown dwarfs, low-mass stars, or halo dark matter. 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, provide us with a unique opportunity for testing alternatives to a BH. 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 timescale arguments, we show that a central cluster is firmly ruled out, unless the cluster consists of extremely dense objects with mass 0.03 M☉ (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 98% of the mass enclosed within the inner edge of the masering disk (3.6 × 107 M☉). A substantial contribution to that mass from a density cusp in the background mass distribution is excluded.
All Related Versions