4.5. Star Formation Histories
Local Group dwarf galaxies vary widely in their star formation histories, chemical enrichment, and age distribution; even within the same morphological type. Despite their individual differences, however, they tend to follow common global relations between, e.g., mean metallicity, absolute magnitude, and central surface brightness. Galaxy mass as well as external effects such as tides appear to play major roles in their evolution.
Sufficiently massive irregulars and dIrrs exhibit continuous star formation at a variable rate. They can continue to form stars over a Hubble time and undergo gradual enrichment. Galaxies such as the LMC (Holtzman et al. 1999, Olsen 1999), SMC, and WLM (Dolphin 2000) have formed stars continuously and experienced considerable chemical enrichment spanning more than 1 dex in [Fe/H]. Their star formation rate, on the other hand, varied and shows long periods of low activity. Interestingly, in the LMC cluster and field star formation activity show little correlation.
Low-mass dIrrs and dSphs often show continuous star formation rates with decreasing star formation rates. They typically show dominant old (or intermediate-age) populations with little or no recent activity. A similar evolution appears to have occurred in dEs. DSph companions of the Milky Way tend to have increased fractions of intermediate-age populations with increasing Galactocentric distance, indicating that external effects such as tidal or ram pressure stripping may have affected their star formation history (e.g., van den Bergh 1994). The two closest dSphs to the Milky Way (other than the currently merging Sagittarius dSph) are Draco and Ursa Minor, which are dominated by ancient populations and are also the least massive dSphs known - possibly due to the early influence of Galactic tides, though present-day positions may not reflect early Galactocentric distances, and reliable orbital information is lacking.
The Local Group dwarf galaxy to show the most extreme case of episodic star formation with Gyr-long periods of quiescence and distinct, well-defined subgiant branches is Carina (Smecker-Hane et al. 1994, Hurley-Keller et al. 1998). It is unclear what caused the interruption and subsequent onset of star formation after the long gaps. Also, the apparent lack of chemical enrichment during these star formation episodes is surprising.