Faint dwarf spheroidals are probably the most common type of galaxy in the Universe. However, they contribute only a small fraction of the total mass and luminosity of all galaxies. The number of dwarf spheroidals has been increasing slowly with time, as dwarf irregulars exhaust their supply of interstellar gas. An additional source of new Ir galaxies, which evolve into dSph galaxies, is provided by objects formed in the tidal tails produced during the interactions of giant spirals (Dottori, Mirabel & Duc 1994). However such “young” dSph galaxies will have low M/L ratios because dark matter that is pulled out of giant galaxies is stretched (Kormendy 1998). As a result fragments pulled out of more luminous galaxies will have a lower density than their progenitors. Furthermore, the initial dark matter density in giants was already lower than it is in dwarf galaxies, even before it was tidally stretched. Finally, dSph galaxies formed from gas that was pulled out of giant galaxies are expected to be more metal rich than most dwarf spheroidals.

The data in Table 2.1 show that of the 18 known Local Group galaxies fainter than MV = –14.0, 13 (72%) are dSph, two (11%) are dIr, two (11%) are of intermediate (dSph/dIr) type, and one (6%) is of unknown type. Excellent reviews on these faint galaxies have been given by Da Costa (1998) and by Mateo (1998).

The dwarf spheroidal Leo I

Leo I was discovered by Harrington (see Wilson 1955) on plates of the Palomar Sky Survey. The Leo I system is located close to the first magnitude star α Leonis, so that it is sometimes referred to as the Regulus system.