  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 Sersic and one Nuker components. P1
and P2 appear to be embedded inside an intermediate component (r_e=3.2") that
is nearly spherical (q=0.97+/-m0.02), while the main galaxy bulge is more
elliptical (q=0.81+/-0.01). The spherical bulge mass of 2.8x10^7 M_sol is
comparable to the supermassive black hole mass (3x10^7 M_sol). 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\arcsec of P2, the
relative mass fraction of the nuclear components is M_BH:M_bulge: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 ~ 2.1x10^7
M_sol, 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.
