The Evolutionary History of Local Group Irregular Galaxies

Abstract

Irregular (Irr) galaxies are gas-rich objects with recent or ongoing star formation. In 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 dwarf Irrs (dIrrs) star formation occurs more quiescently, and the formation of OB associations is common. Irrs and dIrrs tend to experience continuous star formation with amplitude variations and to continue to form stars for another Hubble time. Irrs exhibit lower effective yields than spirals, and [alpha/Fe] ratios below the solar value. This may be indicative of fewer Type II SNe and lower astration rates in their past. Alternatively, many metals may be lost from the shallow galaxy potential wells 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) further supports 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 these dIrrs probably have not yet experienced significant interactions or galaxy harassment. (Abridged)