2001ApJ...550L.151L

The hypothesized supermassive black hole in the nucleus of M31 (M31*) has many features in common with Sgr A* at the Galactic center, yet they differ in several significant and important ways. Although M31* is probably 10 times heavier, its radio luminosity at 3.6 cm is only one-third that of Sgr A*. At the same time, M31* is apparently thousands of times more luminous in X-rays than its Galactic center counterpart. Thus, a comparative study of these objects can be valuable in helping us to understand the underlying physical basis for their activity. We show here that the accretion model being developed for Sgr A* comprises two branches of solutions, distinguished by the relative importance of cooling compared to compressional heating at the radius r_C where the ambient gas is captured by the black hole. For typical conditions in the interstellar medium, the initial temperature [T(r_C)∼10⁶-10⁷ K] sits on the unstable branch of the cooling function. Depending on the actual value of T(r_C) and the accretion rate, the plasma settles onto either a hot branch (attaining a temperature as high as 10¹⁰ K or so at small radii) or a cold branch, in which T drops to ∼10⁴ K. Sgr A* is presumably a ``hot'' black hole. We show here that Very Large Array, Hubble Space Telescope, and Chandra observations of M31* reveal it to be a member of the ``cold'' black hole family. We discuss several predicted features in the spectrum of M31* that may be testable by future multiwavelength observations, including the presence of a prominent UV spike (from hydrogen line emission) that would be absent on the hot branch.