2. INTRODUCTION - DWARF GALAXIES

I will introduce this section of the review by referring to the absolute magnitude vs. central surface-brightness plot that at this conference was referred to as the Binggeli diagram. As Figure 1 of Binggeli's 1994 review paper [5, see also 14, 15] shows, stellar systems generally fall into four distinct regions on this diagram:
(1) Elliptical galaxies and bulges. These form a sequence of decreasing central surface-brightness with increasing luminosity. Absolute magnitudes range from about M_B ~ - 25 (e.g. cD galaxies in the centers of clusters like NGC 6166 in Abell 2199, which have faint central surface-brightnesses) to M_B ~ - 15 (compact low-luminosity ellipticals like M32 which have bright central surface-brightnesses).
(2) Globular clusters. These have high central surface-brightnesses (µ_B 18 mag arcsec^-2), like ellipticals and bulges but have much fainter absolute magnitudes (M_B > - 10). These form a sequence of higher central surface-brightness with increasing luminosity, which is in the opposite sense to the correlation for ellipticals and bulges.
(3) Disks of spirals and S0 galaxies. These have absolute magnitudes -21 < M_B < - 17, and central surface-brightnesses close to 21.6 B mag arcsec^-2 (Freeman's law [11]), which is fainter than the central surface-brightness of almost any sequence (1) ellipticals. At faint magnitudes M_B > - 18, this sequence blends into the dwarf galaxy sequence - see (4) below.
(4) Dwarf Galaxies. These have M_B > - 18 and form a sequence of fainter central surface-brightness with decreasing total luminosity. They are also increasingly dark matter dominated at fainter magnitudes [25, 16]: the faintest dwarfs like Draco and Ursa Minor have mass-to-light ratios ~ 10² within their core-fitting radii [1] and total mass-to-light ratios that are probably much larger than this [28]. It is because of their high dark matter content that a link between the mass functions of these faint dwarfs and the power spectrum of primordial fluctuations on small scales is suggested. Dwarf galaxies are typically one of two morphological types: (i) dwarf irregulars (dIrr) or (ii) dwarf spheroidals (dSph, alternatively called dwarf ellipticals, or dE). These types of galaxies have similar scaling laws (they occupy the same region on the Binggeli diagram), but dwarf irregulars have bluer colors and more disturbed morphologies. Familiar examples in the Local Group are the LMC (dIrr), NGC 205 (a bright dSph with M_B ~ - 15), and Draco (a faint low surface-brightness dSph with M_B ~ - 8).

There do exist other, rare galaxies which do not fit into the classifications above. Examples are huge low surface-brightness giants like Malin 1, and blue compact dwarfs, which appear to have been caught in a short burst of extreme star formation. Throughout the rest of this paper, by "dwarf galaxy", I will mean sequence (4) objects.