Planetary camera observations of the double nucleus of M31

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Abstract

V- and /-band HST Planetary Camera images of the great spiral galaxy in Andromeda, M31, show that its inner nucleus consists of two components separated by 0".49. The outer isophotes of the nucleus at 1".4 < r < 3".0 are elongated, but are concentric with the M31 bulge. The nuclear component with the lower surface brightness, P2, is also coincident with the bulge photocenter to ~0".05; we argue that it is at the kinematic center of the galaxy. The brighter nuclear component, Pl, is well resolved and
\ncorresponds to the nuclear core imaged by Stratoscope II. The central V-band luminosity density for Pl is 5 X 10^4L_☉pc^(-3). P2 is highly elongated and has a shallow cusp. Its central V-band luminosity density is > 2 X 10^4 L_☉ pc^(-3). The total nucleus cannot be modeled as the superposition of two systems each with elliptical and concentric isophotes. The dark zone between the two peaks is deep and is either evidence for strong dust absorption or that the nuclear components are steeply truncated in this region. No strong changes in the V-I color are observed over the nucleus, however. Thus if dust absorption
\ngenerates the asymmetric nuclear morphology, the dust grain size must be unusually large or the dust optical depth must be extremely high. Alternatively the higher surface brightness, off-center nuclear component Pl may be a separate stellar system. The nuclear velocity dispersion profiles of Dressler & Richstone [ApJ, 324, 701 (1988)] and Kormendy [ApJ, 325, 128 (1988)] already show that Pl cannot
\nbe a dynamically cold system such as a globular cluster. Requiring Pl to survive destruction by a central black hole in M31 may also lead to lower limits on its mass-to-light ratio. This hypothesis requires us to be looking at M31 at a special time; however, the lifetime of Pl against orbital decay may be substantially longer than naive dynamical friction arguments would indicate. Under this picture, we
\nargue that P 1 is more likely to be a cannibalized galactic nucleus, possibly having its own central black hole.

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