The Solar Motion Relative to the Local Group

Abstract

New data on the membership of the Local Group, in conjunction with new and improved radial velocity data, are used to refine the derivation of the motion of the Sun relative to the Local Group (hereafter LG). The Sun is found to be moving 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 LG galaxies is bimodal. The LG radial velocity dispersion is 61 \pm 8 km/s. We use various mass estimators to compute the mass of the Local Group and the Andromeda 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 radius of an idealized LG zero-velocity surface is r_0 = 1.18 \pm 0.15 Mpc. The Local Group 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 of low surface brightness dwarfs, this suggests that the Local Group is more compact, and isolated from its surroundings, than previously believed.