The local group of galaxies

Abstract

Hubble's (1936, p. 125) 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 Local Group is found to be Rh ≈ 350 kpc. The zero-velocity surface, which separates the Local Group from the field that is expanding with the Hubble flow, has a radius Ro = 1.18 ± 0.15 Mpc. The total mass of the LG is MLG = (2.3 ± 0.6) × 1012 M⊙. Most of this mass appears to be concentrated in the Andromeda and Milky Way subgroups of the LG. The total luminosity of the Local Group is found to be Mv = 22.0:. This yields a mass-to-light ratio (in solar units) of M/Lv = 44 ± 12. The solar motion with respect to the LG is 306 ± 18 kms-1, directed towards an apex at ℓ = 99° ± 5°, and b = -4° ± 4°. The velocity dispersion within the LG is σr = 61 ± 8 kms-1. The galaxies NGC 3109, Antlia, Sextans A and Sextans B appear to form a distinct grouping with Vr = +114 ± 12 kpc relative to the LG, that is located beyond the LG zero-velocity surface at a distance of 1.7 Mpc from the Local Group centroid. The luminosity distribution of the LG has a slope α = -1.1 ± 0.1. This value is significantly less negative than that which is found in rich clusters of galaxies. 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 subclusters of the Local Group, 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, appear to have shrunk with time. This result is unexpected because Hubble Space Telescope observations appear to show galaxies at z ∼ 3 to be smaller than they are at z = 0. In the Large Magellanic Cloud 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. None of the dwarf galaxies in the LG appears to have experienced a starburst strong enough to have produced a "boojum".