Planetary camera observations of the double nucleus of M31

Abstract

Vand /-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 corresponds to the nuclear core imaged by Stratoscope II. The central V-band luminosity density for Pl is 5 X 10 4 L)p pc-3 • P2 is highly elongated and has a shallow cusp. Its central V-band luminosity density is > 2 X 10 L 0 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 generates 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 be 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 argue that P 1 is more likely to be a cannibalized galactic nucleus, possibly having its own central black hole.

A wide-field H I mosaic of Messier 31

“…At optical wavelengths (in the R-and Vband) Walterbos & Kennicutt (1988) found a bulge effective radius of 2.3 kpc and a bulge to disk luminosity ratio B/D = 0.82. Using the same dataset in the R-band combined with Hubble Space Telescope data of Lauer et al (1993) and data from Kent (1983) for the innermost regions Geehan et al (2006) derived a much smaller bulge scale radius: 0.61 kpc (similar to what can be inferred by inspecting the 2MASS images). Using the 3.6 μm Spitzer image Barmby et al (2006) modelled the bulge light distribution with a R 1/4 de Vaucouleurs law and found a 1.7 kpc effective radius and a bulge to disk light ratio of B/D = 0.78.…”

“…M 31 is known to have a complex merging history. Its multiple nucleus (Lauer et al 1993) and the extended stellar stream and halo (e.g. Irwin et al 2001;Chapman et al 2008) are clear signs of a tumultuous life.…”

The dynamical evolution of massive black hole binaries I. Hardening in a fixed stellar background

“…Multiple mergers of bh binaries can lead to the buildup of massive central bhs (Hut and Rees, 1992), although if the binaries do not merge quickly the bhs can be ejected through three-body interactions (Xu and Ostriker, 1994;Valtonen, 1996), a process that has been proposed to explain the double radio lobes of active galaxies (Saslaw et al, 1974;Valtonen et al 1994). The double nucleus of the nearby galaxy M31 (Lauer et al, 1993) could be a bh binary in the making, although Tremaine (1995) has proposed a plausible model that requires only one bh. If binary mergers are frequent they will be the most interesting source of gravitational radiation for the planned space-based detector LISA (Haehnelt, 1994;Bender et al, 1995).…”

The M 31 double nucleus probed with OASIS and HST

“…Upgrading the modeling to an axisymmetric (or triaxial) geometry would have been possible, but it was already known since the Stratoscope II observations (Light et al 1974), that the light distribution was not even symmetric. This asymmetry was resolved in an intriguing double structure by Lauer et al (1993) using the HST (pre-COSTAR) WFPC imaging capabilities. In the HST original and post-COSTAR images (Lauer et al 1998), the double nucleus appears as a bright peak (P1) offset by ∼0.…”