Introduction

Since its existence was first noted by Edwin Hubble, the Local Group has been the subject of continuing exploration. Hubble (1936, p. 125) refers to it as “a typical, small group of nebulae which is isolated in the general field.” The elongated core of the Local Group, consisting of the Andromeda galaxy, the Milky Way, and their close companions does, indeed, form a rather well-isolated group. However, the low-density outer envelope of the Local Group may mingle with the coronae of other nearby clusterings, such as the South Polar group and the Mafei/IC 342 group. As was already noted in Section 18.1 the velocity dispersion of Local Group galaxies establishes a rather well-determined radius of 1.2 Mpc for the zero-velocity surface of the Local Group. If we adopt MV (Sun) = +4.82 ± 0.02 (Hayes 1985), the total luminosity of the Local Group galaxies listed in Table 19.1 is found to be 4.2 × 1010L⊙(V). Of this amount 86% is provided by M31 and the Galaxy. The discovery of additional faint Local Group members would not increase the estimated total luminosity of the Local Group significantly. Comparison with the Local Group mass of M = (2.3±0.6)× 1012M⊙ yields M/LV = 44±12. This shows that dark matter in the Local Group outweighs visible matter by about an order of magnitude. Data on the mass-to-light ratios of the Galactic and Andromeda subgroups of the Local Group are given in Table 20.1.

Local Group calibrators

Modern research has vindicated Hubble's expectation that Local Group members (“these neighboring systems”) would furnish a small sample collection of nebulae, from which criteria might be developed to explore the remoter regions of space.