4.2. Old Populations
A common property of all Local Group dwarfs studied in detail is the existence of an old population, whose presence can be inferred either from HB stars and/or from photometry reaching below the oldest main-sequence turnoff. Old populations may be difficult to detect in the central portions of galaxies with significant intermediate-age or young populations, as the location of these stars in a CMD may obscure an old HB. Also, coverage of only a small field of view may be insufficient to reliably detect a sparsely populated HB (compare the findings of Gallart et al. 1999 and GHeld et al. 2000 for Leo I). Age dating of the oldest populations is reliably possible only where high-quality photometry well below the oldest main-sequence turnoff exists; a challenge for present-day telescopes already for galaxies at the distance of M31. Definite statements about the existence of an old population are possible only where the photometry reaches at least the HB; feasible in principle with present-day telescopes out to distances 3 Mpc.
Deep main-sequence photometry based largely on Hubble Space Telescope data revealed that the ages of the oldest populations in the LMC (Holtzman et al. 1999), Sagittarius (Layden & Sarajedini 2000), Draco, Ursa Minor (Feltzing, Gilmore, & Wyse 1999), Sculptor (Monkiewicz et al. 1999), Carina (Mighell 1997), Fornax (Buonanno et al. 1998), and Leo II (Mighell & Rich 1996) are as old as the oldest Galactic globular clusters and bulge populations. Thus all of these galaxies share a common epoch of early star formation. Similarly old ages were inferred from the existence of blue HBs in Sextans (Harbeck et al. 2000), Leo I (Held et al. 2000), Phoenix (Smith, Holtzman, & Grillmair 2000), IC 1613 (Cole et al. 1999), Cetus (Tolstoy et al. 2000), And I (Da Costa et al. 1996), And II (Da Costa et al. 2000), NGC 185 (Geisler et al. 1999), NGC 147 (Han et al. 1997), Tucana (Lavery et al. 1996), M31 (Ajhar et al. 1996), potentially in M32 (Brown et al. 2000), and spectroscopically for one of NGC 6822's globular clusters (Cohen & Blakeslee 1998). Assuming that age is the second parameter determining HB morphology the apparent lack of a blue HB in M33 globular clusters (Sarajedini et al. 1998) and in the field populations of WLM (Dolphin 2000), Leo A (Tolstoy et al. 1998), DDO 210 (Tolstoy et al. 2000) and the Small Magellanic Cloud (SMC) may be interpreted as evidence for delayed formation of the majority of the old population in these galaxies. Furthermore, the oldest globular cluster in the SMC, NGC 121, is a few Gyr younger than the oldest Galactic globulars (Shara et al. 1998) A complete lack of an old population has so far not been established in any Local Group galaxy.