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Date and Time of the Query: 2019-05-27 T04:30:11 PDT
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For refcode 1999A&ARv...9..273V:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
1999A&ARv...9..273V The local group of galaxies Sidney van den Bergh National Research Council of Canada, Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics, 5071 West Saanich Road. Victoria, British Columbia. V8X 4M6. Canada (e-mail: vdb@dao.nrc.caj Received 14 April 1999 Summary. 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 modem 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 R_h_ ~ 350 kpc. The zero-velocity surface, which separates the Local Group from the field that is expanding with the hubble flow, has a radius R_0_ = 1.18 +/- 0.15 Mpc. The total mass of the LG is M_LG_ = (2.3 +/- 0.6) x 10^12^ M_sun_. 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 M_V_ = -22.0: This yields a mass-to-light ratio (in solar units) of M/L_v_ = 44 +/- 12. The solar motion with respect to the LG is 306 +/- 18 kms^-1^, directed towards an apex at l = 99^deg^ +/- 5^deg^, and b = 4^deg^ +/- 4^deg^ The velocity dispersion within the LG is {sigma}_r_ = 61 +/- 8 kms^-1^. The galaxies NGC 3109, Antlia, Sextans A and Sextans B appear to form a distinct grouping with V_r_ = +114 +/- 12kpc relative to the LG, that is located beyond the LG zero-velocity surface at a distance of 13 Mpc from the Local Group centroid. The luminosity distribution of the LG has a slope {alpha} = -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 1 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". Keywords. Galaxies: clusters D Galaxies: Local Group D Galaxies: Magellanic Clouds
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