The ``Local Group'' is the small group of galaxies around the Milky Way and M31. The size of the Local Group is not well known, and its galaxy census is incomplete for low-surface-brightness galaxies. Recent studies suggest that the radius of the zero-velocity surface of the Local Group is ~ 1.2 Mpc (Courteau & van den Bergh 1999) when a spherical potential is assumed. Within this radius 35 galaxies have been detected (see Grebel 2000a for a list). Since information about orbits is lacking it is unknown which ones of the more distant galaxies within and just outside of the adopted Local Group boundaries are actually bound to the Local Group. Many faint Local Group galaxies were only discovered in recent years, and searches are continuing. Hierarchical cold dark matter (CDM) models predict about 10 times more dark matter halos than the number of known Local Group satellites (e.g., Klypin et al. 1999). Compact high-velocity clouds (Braun & Burton 1999), which appear to be dark-matter-dominated with total estimated masses of a few 108 M may be good candidates for the ``missing'' satellites.
The Local Group comprises galaxies with a variety of different morphological types, a range of masses, ages, and metallicities, and differing degrees of isolation. Their proximity makes these galaxies ideal targets for detailed studies of their star formation histories from their resolved stellar populations and of galaxy evolution in general. Furthermore, Local Group galaxies provide a convenient set of targets for studies of the nature of dark matter. Several Local Group reviews have appeared in the past few years, including Mateo (1998), Grebel (1997, 1999, 2000a), and the very detailed recent reviews by van den Bergh (1999, 2000). Reviews dealing with dark matter in Local Group dwarf spheroidals include Mateo (1997) and Olszewski (1998).
Figure 1. A scaled 3-D representation of the Local Group (LG). The dashed ellipsoid marks a radius of 1 Mpc around the LG barycenter (assumed to be at 462 kpc toward l = 121.7 and b = -21.3 following Courteau & van den Bergh 1999). Distances of galaxies from the the arbitrarily chosen plane through the Milky Way are indicated by solid lines (above the plane) and dotted lines (below). Morphological segregation is evident: The dEs and gas-deficient dSphs (light symbols) are closely concentrated around the large spirals (open symbols). DSph/dIrr transition types (e.g., Pegasus, LGS 3, Phoenix) tend to be somewhat more distant. Most dIrrs (dark symbols) are fairly isolated and located at larger distances. Also indicated are the locations of two nearby groups.