8.2 Dwarfs in the Local Group

The provisional and certainly incomplete luminosity function of the Local Group (Section 4) has only six average or giant members (M_T < - 16) and a score of dwarfs fainter than -16, of which six (or seven including the SMC) are Magellanic irregulars in the range -16 to -12 and 15 are spheroidal systems in the range -16 to -9 or fainter. This is a minimum because extreme dwarf ellipticals of the Sculptor-Fornax type are observable only if µ 22 (µ₀ < 21.5 or 0.2 Mpc) since their discovery on the Palomar Sky Survey plates requires m_* 20.5 (if M_* -1.5); the fact that a dozen are known within this range suggests that their space density is high (50 to 100 per Mpc³), unless they are satellites of our Galaxy, perhaps related to globular clusters (Wilson 1955) rather than independent galaxies. Star counts in several of the largest dE systems (Hodge 1960, 1961, 1963, 1964), which indicate tidally limited radii, favor the second alternative. There is only a marginal possibility of detecting such systems at the distance of the Andromeda group even with the largest reflectors (µ 25, m_* 23.5). ⁽⁴⁾ A fortiori such systems are beyond the reach of the largest telescopes even in the nearest groups (µ > 27, m_* > 25.5). For all practical purposes we may be missing the most common type of galaxy in the Universe, much as we fail to detect all except a few of the nearest dwarf stars of M > +10.

To a lesser extent the same remark applies to the dwarf Magellanic irregulars of the IC 0010-IC 1613 type of which at least six are known in the Local Group. The presence of blue supergiants and H II regions, however, makes them more easily detectable and well beyond the Local Group (µ < 30 if m_* < 21 to 22 and M_* -8 to -9).

Small ellipticals of the M32-NGC 0205 type are also observable beyond the Local Group, but their small diameters make them difficult to distinguish from star images at the distance of the Virgo cluster. For instance, M32 with a standard linear diameter D(0) = 0.6 kpc would have an apparent diameter of 0'.2 only at the adopted distance = 12.6 Mpc of the Virgo cluster and would therefore look very much like NGC 4486B which is undistinguishable from star images on survey plates.

⁴ Three such systems were discovered by van den Bergh (1972a) on IIIa-J plates taken with the 48-inch Palomar Schmidt telescope and were resolved into stars (m_* 24) with the 200-inch reflector (van den Bergh 1972b, 1973); the total magnitude of And I is B 14.9 with (B - V) 0.75, (U - B) 0.25 (unpublished McDonald data). Back.