Local Group Dwarf Elliptical Galaxies. I. Mapping the Dynamics of NGC 205 Beyond the Tidal Radius

NGC 205 is the nearest example of a dwarf elliptical (dE) galaxy and theprototype of this enigmatic galaxy class. Photometric evidence suggests thatNGC 205, a close satellite of the M31 galaxy, is tidally interacting with itsparent galaxy. We present stellar radial velocity measurements out to aprojected radius of 20' (5 kpc) in NGC 205 based on Keck/DEIMOS multislitspectroscopic observations of 725 individual red giant branch stars. Ourkinematic measurements extend from the center out to six times the effectiveradius of NGC 205, well past the expected tidal radius. The contamination inour kinematic sample from M31 field stars is estimated to be a few percentbased on maximum likelihood fits. We measure a maximum major-axis rotationspeed for the body of NGC 205 of 11+/- 5 kms; this is the first dE galaxy inwhich the maximum rotation velocity has been measured. Combined with thevelocity dispersion, we conclude that NGC 205 is supported by a combination ofrotation and random motions. At a major-axis distance of 4.5' (1 kpc), thevelocity profile of NGC 205 turns over; stars beyond this radius are movingcounter to the rotation of the inner part of the galaxy. The turnover radius iscoincident with the onset of isophotal twisting and the estimated tidal radius,suggesting that the outer kinematics of NGC 205 are dominated by gravitationalinteractions with the nearby M31 galaxy. The motion of stars beyond a radius of4.5' implies that NGC 205 is in a prograde encounter with its parent galaxyM31.