The Andromeda Galaxy (M31) is, galactically speaking, our own Milky Way's bigger brother. Known to backyard stargazers as the most distant object visible to the naked eye, M31 is also the nearest, and therefore best studied, large spiral.

M31's nucleus (below) has been known to be strange since the early 1970's, when photographs taken by the balloon-borne Stratoscope II showed it to be elongated and oddly asymmetric. Later images from the Hubble Space Telescope resolved the nucleus into a double structure, with the optically brighter peak (P1) lying about 0.5 arcsec from the UV-bright P2.

The favored explanation for the double nucleus, due to Tremaine (1995), is that it results from an eccentric stellar disk surrounding the central supermassive black hole. The structure, dynamics, and stability of such disks are topics of current research.

M31 wide-field mosaic (roughly 3 degrees square) by Robert Gendler.

Stratoscope image from Light et al. (1974)

HST WFPC image from T. Lauer et al. (1998)

Eccentric disk model by R. Salow

We have examined the requirements on the structure of a broad eccentric ring such as that proposed for M31. The ring's self gravity will cause it to rip itself apart by differential precession, unless its orbital structure is such that all orbits precess coherently. We showed (Statler 1999) that a cold ring made solely of periodic orbits would have to have a characteristic eccentricity structure (left) to precess uniformly. The extension of this idea to disks with finite velocity dispersion (Salow & Statler 2001) yields density distributions like that at right.

In projection, the rotation curve (a) and dispersion profile (b) of the model (smooth curves) can produce a fair match to data (crosses, error bars omitted) from the HST Faint Object Camera Spectrograph (Statler et al. 1999) The fit has been optimized only by eye, and a more rigorous fit to all the available data, with a definitive measurement of the central mass, will be forthcoming.

Related Links

Tom Statler's Publications and Preprints
Robert Salow's M31 Page
RETURN to the Astrophysics Group's Home Page
Last updated 31 July 01.