4.2. Intermediate-age and Old Stellar Populations
Irrs and massive dIrrs tend to show extended halos of intermediate-age stars (ages ~ 1 to ~ 10 Gyr), which can be conveniently traced by carbon stars (e.g., Letarte et al. 2002). In the Magellanic Clouds, the density distributions of different populations ages become increasingly more regular and extended with increasing age (e.g., Cioni et al. 2000; Zaritsky et al. 2000), whereas the young populations are responsible for the irregular appearance of these two galaxies. The centroids of the different populations do not always coincide. Features resembling stellar bars are found in many dIrrs, which do not necessarily coincide with the peak HI distribution or its centroid. In low-mass dIrrs there are not enough intermediate-age tracers such as C stars to say much about the distribution of these populations (see, e.g., Battinelli & Demers 2000); the number of C stars decreases with absolute galaxy luminosity and also with galaxy metallicity (see Groenewegen 2002 for a recent review and census of C stars in the Local Group).
All Irr and dIrr galaxies examined in detail so far show clear evidence for the presence of old (> 10 Gyr) populations, a property that they appear to share with all galaxies whose stellar population have been resolved. For instance, deep ground-based imaging of the "halos" of dIrrs led to the detection of old red giant branches (e.g., Minniti & Zijlstra 1996; Minniti, Zijlstra, & Alonso 1999). In closer dIrrs, horizontal branch stars have been detected in field populations (e.g., IC 1613: Cole et al. 1999; Phoenix: Holtzman, Smith, & Grillmair 2000; WLM: Rejkuba et al. 2000; LeoA: Dolphin et al. 2002) and in globular clusters (e.g., WLM: Hodge et al. 1999). Horizontal branch stars are unambiguous tracers of ancient populations. In the closest dIrrs and Irrs even the old main sequence turn-offs have been resolved, allowing differential age dating. Interestingly (with the possible exception of the SMC), the oldest datable populations in all nearby dwarf galaxies turn out to be indistinguishable in age from each other and from the Milky Way, indicating a common epoch of early star formation (e.g., Olsen et al. 1998; Johnson et al. 1999; see Grebel 2000 for a full list of references). Apart from the recent interaction in NGC6822 (Section 3.2), the old populations also are usually the most extended ones. However, their fractions vary: in some cases they only constitute a tiny portion of the stellar content of their parent galaxy.