The nucleus of M31

High-resolution measurements of the rapid rotation and large velocity dispersion in the nucleus of the Andromeda nebula (M31) hint at the possible influence of a massive black hole in that region. The recent detection of an unresolved radio source coincident with the dynamical center of the galaxy has evoked a direct comparison of M31's nucleus with the compact, nonthermal radio source Sgr A* at the center of our own Galaxy. We calculate the broad-band spectrum expected of a 10⁷ M_⊙ black hole accreting from an ambient galactic wind in the nucleus of M31, and we discuss several testable predictions of this model: (1) when resolved, this source will probably be elongated, perhaps in the direction of the semistellar nucleus BD +40° 148; (2) its radio spectral index is expected to be ∼0.4 (compared with ∼0.25 for Sgr A*); (3) the radio component should steepen sharply between ∼10¹⁰ and 10¹¹ Hz; (4) if an association between this radio source and a nearby Einstein X-ray source is confirmed, its radio luminosity should be variable on a time of ≲1 yr; and (5) the variability of its radio and X-ray fluxes should be anticorrelated.