2. LOCAL GROUP GALAXY CONTENT AND DISTRIBUTION

The most massive and most luminous Local Group galaxies are the two spirals Milky Way and M31 ( 10¹² M, M_V -21 mag). The third, less luminous and less massive Local Group spiral M33 does not have any known companions and belongs to the M31 subsystem. About two thirds of the Local Group galaxies are found within 300 kpc of the two spirals. The majority of these close companions are dwarf spheroidal and dwarf elliptical galaxies. The ensemble of dwarf irregular galaxies, on the other hand, shows little concentration toward the two large spirals (although the two most massive Local Group irregulars, the Large and the Small Magellanic Cloud (LMC and SMC), are close neighbors of the Milky Way and interact with it as well as with each other). This correlation between morphological type and distance from massive galaxies is also known as morphological segregation and may be to some extent a consequence of evolutionary effects.

Whether a galaxy should be considered a dwarf galaxy is somewhat arbitrary, and different authors use different criteria. For the purpose of this review all galaxies with M_V > -18 mag will be considered dwarf galaxies, which results in 31 dwarfs, excluding only the three spirals and the LMC. We distinguish the following basic types of dwarf galaxies in the Local Group:

• Dwarf irregulars (dIrrs) with M_V -18 mag, µ_V 23 mag arcsec^-2, R 5 kpc, M_{H I} 10⁹ M, and M_tot 10¹⁰ M. DIrrs are irregular in their optical appearance, gas-rich, and show current or recent star formation. Several of the dIrrs contain globular or open clusters.

• Dwarf ellipticals (dEs) with M_V -17 mag, µ_V 21 mag arcsec^-2, R 4 kpc, M_{H I} 10⁸ M, and M_tot 10⁹ M. DEs look globular-cluster-like in their visual appearance with a pronounced central concentration. All dEs are companions of M31. Two of the four dEs (M32, NGC 205) are nucleated. M32, a dE very close to M31, has a central black hole and follows the same scaling relations as large elliptical galaxies, whereas the other dEs resemble dSphs and are therefore called spheroidals by van den Bergh (1999, 2000). All dEs except for M32 contain globular clusters.

• Dwarf spheroidals (dSphs) with M_V -14 mag, µ_V 22 mag arcsec^-2, R 3 kpc, M_{H I} 10⁵ M, and M_tot ~ 10⁷ M. These galaxies show very little central concentration and are dominated by old and intermediate-age stellar populations. Only three (Sgr, For, And I) contain globular clusters. With the exception of two isolated dSphs (Tuc and Cet) all known dSphs are close neighbors of M31 or the Milky Way. DSphs are gas-poor systems. Sensitive searches for H I in dSphs yielded only low upper limits, but recent studies detected extended H I clouds in the surroundings of some dSphs that may be associated with them judging from the similarity of their radial velocities (Carignan et al. 1998, Blitz & Robishaw 2000).

A few dwarf galaxies (Phe, LGS 3) are classified as ``transition-type'' objects and may be evolving from low-mass dIrrs to dSphs. These dIrr/dSph galaxies are found at distances of 250 kpc < D_Spiral < 450 kpc. The Local Group does not contain blue compact dwarf galaxies, dwarf spirals, or massive ellipticals.