The Evolutionary History of Local Group Irregular Galaxies

Irregular (Irr) and dwarf irregular (dIrr) galaxies are gas-rich galaxies with recent or ongoing star formation. In the absence of spiral density waves, star formation occurs largely stochastically. The scattered star-forming regions tend to be long-lived and migrate slowly. Older populations have a spatially more extended and regular distribution. In fast-rotating Irrs high star formation rates with stronger concentration toward the galaxies' center are observed, and cluster formation is facilitated. In slowly or nonrotating dIrrs star formation regions are more widely distributed, star formation occurs more quiescently, and the formation of OB associations is common. On average, Irrs and dIrrs are experiencing continuous star formation with amplitude variations and can continue to form stars for another Hubble time.

Irrs and dIrrs exhibit lower effective yields than spirals, and [α/Fe] ratios below the solar value. This may be indicative of fewer Type II supernovae and lower astration rates in their past (supported by their low present-day star formation rates). Alternatively, many metals may be lost from the shallow potential wells of these galaxies due to selective winds. The differences in the metallicity-luminosity relation between dIrrs and dwarf spheroidals (which, despite their lower masses, tend to have too high a metallicity for their luminosity as compared to dIrrs) lends further support to the idea of slow astration and slow enrichment in dIrrs. The current data on age-metallicity relations are still too sparse to distinguish between infall, leaky-box, and closed-box models. The preferred location of dIrrs in the outer parts of galaxy groups and clusters and in the field as well as the positive correlation between gas content and distance from massive galaxies indicate that most of the dIrrs observed today probably have not yet experienced significant interactions or galaxy harassment.