Assessing the Formation Scenarios for the Double Nucleus of M31 Using Two-dimensional Image Decomposition

The double nucleus geometry of M31 is currently best explained by the eccentric disk hypothesis of Tremaine, but whether the eccentric disk resulted from the tidal disruption of an inbounding star cluster by a nuclear black hole, or by an m = 1 perturbation of a native nuclear disk, remains debatable. I perform detailed 2-D decomposition of the M31 double nucleus in the Hubble Space Telescope V -band to study the bulge structure and to address competing formation scenarios of the eccentric disk. I deblend the double nucleus (P1 and P2) and the bulge simultaneously using five Sérsic and one Nuker components. P1 and P2 appear to be embedded inside an intermediate component (re = 3. 2) that is nearly spherical (q = 0.97± 0.02), while the main galaxy bulge is more elliptical (q = 0.81± 0.01). The spherical bulge mass (2.8 × 10M⊙), being coincident with the supermassive black hole mass (3 × 10M⊙), conjoined with a shallow bulge cusp, are consistent with the scenario that the bulge was scoured by spiraling binary supermassive black holes. In the 2-D decomposition, the bulge is consistent with being centered near the UV peak of P2, but the exact position is difficult to pinpoint because of dust in the bulge. P1 and P2 are comparable in mass. Within a radius r = 1 of P2, the relative mass fraction of the nuclear components is M• : Mbulge : P1 : P2 = 4.3 : 1.2 : 1 : 0.7, assuming the luminous components have a common mass-to-light ratio of 5.7. The eccentric disk as a whole (P1+P2) is massive,M ≈ 2.1×10M⊙, comparable to the black hole and the local bulge mass. As such, the eccentric disk could not have been formed entirely out of stars that were stripped from an inbounding star cluster. Hence, the more favored scenario is that of a disk formed in situ by an m = 1 perturbation, caused possibly by the passing of a giant molecular cloud, or the passing/accretion of a small globular cluster. Subject headings: fundamental parameters – technique: photometry – galaxies: structure, nucleus – individual: M31