The Local Group, our immediate cosmic neighborhood, resembles other nearby galaxy groups in many ways, including in its galaxy content, structure, mass, and other properties (e.g., Karachentsev et al. 2002a, b). It is our best local laboratory to study galaxy evolution at the highest possible resolution and in the greatest possible detail. The Local Group contains two dominant spiral galaxies surrounded by a large number of smaller galaxies. Thirty-six galaxies are currently believed to be members of the Local Group if a zero-velocity surface of 1.2 Mpc is adopted (Courteau & van den Bergh 1999; Grebel, Gallagher, & Harbeck 2003) (1). The smaller galaxies in the Local Group include a spiral galaxy (M33), 11 gas-rich Irr and dIrr galaxies (including low-mass, so-called transition-type galaxies that comprise properties of both dIrrs and dwarf spheroidals), four elliptical and dwarf elliptical galaxies, and 17 gas-deficient dwarf spheroidal (dSph) galaxies. For a listing of the basic properties of these galaxies, see Grebel et al. (2003). Their three-dimensional distribution is illustrated in Grebel (1999; Fig. 3). Recent reviews of Local Group galaxies include Grebel (1997, 1999, 2000), Mateo (1998), and van den Bergh (1999, 2000).
DIrrs are the second most numerous galaxy type in the Local Group. While new dwarf members of the Local Group are still being discovered (e.g., Whiting, Hau, & Irwin 1999), these tend to be gas-deficient, low-mass dSph galaxies, which have intrinsically low optical surface brightnesses and cannot be found from their HI 21 cm emission lines. The Irr and dIrr census of the Local Group appears to be complete.
Irrs and dIrrs are found in galaxy groups and clusters as well as in the field and exhibit little concentration toward massive galaxies in contrast to early-type dwarfs. This morphological segregation is clearly seen in the Local Group and in nearby groups (Fig. 1). It becomes even more pronounced in galaxy clusters, where the distribution of Irrs shows the least concentration of all galaxy types toward the cluster core (e.g., Conselice, Gallagher, & Wyse 2001, and references therein), which has been attributed to continuing infall of Irrs and subsequent harassment. Conversely, in very loose groups or "clouds" (such as the Canes Venatici I Cloud) that are still far from approaching dynamical equilibrium, an overabundance of Irrs and dIrrs is observed as compared to early-type dwarfs (Karachentsev et al. 2003a), indicative of a lack of interactions.
Figure 1. Morphological segregation in the Local Group (filled histograms; see Grebel 2000) and in the M81 and Cen A groups (dashed histograms; input data from Karachentsev et al. 2002a, b). Note the pronounced concentration of gas-poor, early-type dwarfs around the nearest massive primary galaxy, while the gas-rich, late-type dwarfs show less concentration and are more widely distributed. This may be a signature of the impact of environmental effects, such as gas stripping.
1 Note that recent kinematic estimates suggest an even smaller radius of (0.94 ± 0.10) Mpc for the zero-velocity surface (Karachentsev et al. 2002c), which reduces the above number of Local Group dwarf galaxies by two. Back.