Merging between a Central Massive Black Hole and a Compact Stellar System: A Clue to the Origin of M31'S Nucleus

Abstract

The central bulge of M31 is observed to have two distinct brightness peaks with the separation of $\sim$ 2 pc. Tremaine (1995) recently proposed a new idea that the M31's nucleus is actually a single thick eccentric disk surrounding the central super-massive black hole. In order to explore the origin of the proposed eccentric disk, we numerically investigate the dynamical evolution of a merger between a central massive black hole with the mass of $\sim$ $10^7$ $M_{\odot}$ and a compact stellar system with the mass of $\sim$ $10^6$ $M_{\odot}$ and the size of a few pc in the central 10 pc of a galactic bulge. We found that the stellar system is destroyed by strong tidal field of the massive black hole and consequently forms a rotating nuclear thick stellar disk. The orbit of each stellar component in the developed disk is rather eccentric with the mean eccentricity of $\sim$ 0.5. These results imply that the M31's nuclear eccentric disk proposed by Tremaine (1995) can be formed by merging between a central massive black hole and a compact stellar system. We furthermore discuss when and how a compact stellar system is transferred into the nuclear region around a massive black hole.