The Solar Motion Relative to the Local Group

New data on the membership of the Local Group, in conjunction with new andimproved radial velocity data, are used to refine the derivation of the motionof the Sun relative to the Local Group (hereafter LG). The Sun is found to bemoving with a velocity of Vbulk = 306 \pm 18 km/s, towards an apex at l = 99\pm 5 and b = -4 \pm 4 degrees. This agrees very well with previous analyses,but we discuss the possibility of a bias if the phase-space distribution of LGgalaxies is bimodal. The LG radial velocity dispersion is 61 \pm 8 km/s. We usevarious mass estimators to compute the mass of the Local Group and theAndromeda subgroup. We find M_LG = (2.3 \pm 0.6)*10^12 \Msol, from which M/L_V= 44 \pm 12 (in solar units). For an assumed LG age of 14 \pm 2 Gyr, the radiusof an idealized LG zero-velocity surface is r_0 = 1.18 \pm 0.15 Mpc. The LocalGroup is found to have 35 likely members. Only three of those have (uncertain)distances \grtapr 1.0 Mpc from the LG barycenter. Barring new discoveries oflow surface brightness dwarfs, this suggests that the Local Group is morecompact, and isolated from its surroundings, than previously believed.Comment: To appear in the Astronomical Journal, July 1999 -- 23 pages, 1 table (landscape format), and 6 figure