The Solar Motion Relative to the Local Group

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Abstract
New data on the membership of the Local Group (LG) are used, in conjunction with new and improved radial velocity data, to refine the derivation of the motion of the Sun relative to the LG. The Sun is found to be moving with a velocity of V = 306 ± 18 km s-1 toward an apex at l = 99° ± 5° and b = -4° ± 4°. This finding 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 ± 8 km s-1. We use various mass estimators to compute the mass of the LG and the Andromeda subgroup. We find MLG = (2.3 ± 0.6) × 1012 M⊙, from which M/LV = 44 ± 12 (in solar units). For an assumed LG age of 14 ± 2 Gyr, the radius of an idealized LG zero-velocity surface is r0 = 1.18 ± 0.15 Mpc. The LG is found to have 35 likely members. Only three of these have (uncertain) distances 1.0 Mpc from the LG barycenter. Barring new discoveries of low surface brightness dwarfs, this suggests that the LG is more compact and more isolated from its surroundings than previously believed.
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