AAS 197, January 2001
Session 28. Kinematics, ISM and Star Formation in the Local Group
Oral, Monday, January 8, 2001, 1:30-3:00pm, Pacific One

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[28.04] Galaxy Interactions in the Andromeda Sub-Group: M31, M32 & NGC~205

P.I. Choi, P. Guhathakurta (UCSC), K.V. Johnston (Wesleyan University)

We investigate the interaction of Local Group dwarf galaxies M32 & NGC~205 with their parent galaxy M31. A 2\circ\times5\circ mosaic of KPNO Burrell Schmidt 20482 CCD images is used to obtain surface photometry of M31 and its satellites in the B and I bands. The M31 background is modeled and removed using standard ellipse fitting techniques. The satellites are measured down to a limiting isophote of 27~mag~arcsec-2 in the B band and 25~mag~arcsec-2 in the I band. These limiting isophotes correspond to semi-major axis lengths of 420'' (1.6~kpc) and 720'' (2.7~kpc) for M32 and NGC~205, respectively.

We confirm earlier hints of excess light in the outskirts of M32: clear evidence is seen for a sharp (upward) break in the radial surface brightness profile at r=2.5' relative to the r\rm eff=30'' (0.11~kpc) de~Vaucouleurs profile which fits the inner portion of M32. A more subtle, previously undetected (downward) break is also detected at r=1'. Tests show that the excess: (1)~is insensitive to details of background subtraction; and (2)~has the same B-I color as M32's interior and significantly different from the colors of typical M31 residual spiral arm/dust lane features. The breaks in M32's profile are accompanied by sharp changes in ellipticity.

NGC~205 displays severe isophote twisting (\Delta\theta\rm pa~0\circ) between r=2.5'--12'. This is accompanied by a subtle downward break in the brightness profile at r=2.5', relative to the extrapolation of the r\rm scale=150''--170'' (0.56--0.63~kpc) exponential laws that fit the inner parts of NGC~205 (75''--300'').

The isophotal distortion and profiles features of M32 and NGC~205 are the same across B, R and I. To better constrain the nature of the interaction, surface brightness measurements are being complemented by N-body simulations and Keck/ESI spectroscopic measurements of the mean velocity and dispersion of ~0 red giant branch stars in the vicinity of the profile break in M32 (1.5'

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