The Local Group of Galaxies

Abstract

Hubble's view that the Local Group (LG) is "a typical, small group of nebulae which is isolated in the general field" is confirmed by modern data. The total number of certain and probable Group members presently stands at 35. The half- mass radius of the LG is Rh = 350 kpc, and the zero-velocity surface has a radius Ro = 1.18 +/- 0.15 Mpc. The total mass and mass-to-light ratio (in solar units) of the LG are MLG = (2.3 +/- 0.6) x 10^12 Msol and 44 +/- 12, respect. The solar motion with respect to the LG is 306 +/- 18 km/s, directed towards an apex at (l,b)=(99 +/-5, -4 +/- 4). The velocity dispersion within the LG is sigma = 61 +/- 8 km/s. The luminosity distribution of the LG has a slope -1.1 +/- 0.1. This value is significantly less negative than that for rich galaxy clusters. The luminosity distribution of the dwarf spheroidal galaxies is steeper than that for dwarf irregulars. Furthermore the dSph galaxies are strongly concentrated within the Andromeda and Milky Way sub-clusters of the LG, whereas the majority of dIr galaxies appear to be free-floating members of the LG as a whole. With the possible exception of Leo I and Leo A, most LG members appear to have started forming stars simultaneously ~15 Gyr ago. Many of the galaxies, for which evolutionary data are available, seem to have shrunk with time. This result is unexpected because HST observations appear to show galaxies at z ~3 to be smaller than they are at z=0. In the LMC the rate of cluster formation was low for a period that extended from ~12 Gyr to ~4 Gyr ago. The rate of cluster formation may have increased more rapidly 3-5 Gyr ago, than did the rate of star formation. The reason for the sudden burst of cluster formation in the LMC ~4 Gyr ago remains obscure. [abridged]