The dwarf galaxies in the Local Group vary widely in their star formation and enrichment histories, times and duration of their major star formation episodes, and fractional distribution of ages and subpopulations. Indeed, when studied in detail, no two dwarf galaxies turn out to be alike, not even if they are of the same morphological type or have similar luminosities (Grebel 1997). On the other hand, in spite of their individual differences, they do follow certain common global correlations such as increasing mean metallicity with luminosity (Section 6.2).
4.1. Large-scale Star Formation
The ISM properties of Irrs and dIrrs outlined in the previous section already show that there are spatial variations in star formation history and other characteristics within these galaxies. In general, dwarf galaxies of all types show a tendency for the younger populations to be more centrally concentrated (and possibly more chemically enriched), whereas older populations are more extended (Grebel 1999, 2000; Harbeck et al. 2001). In Irr and dIrr galaxies, HII regions tend to be located within the part of the galaxy that shows solid body rotation and are usually even more centrally concentrated (Roye & Hunter 2000). Star-forming regions may, however, be found out to six optical scale lengths, indicating that star formation is truncated at lower gas density thresholds than in spirals (Parodi & Binggeli 2003). In dIrrs dominated by chaotic motions, the degree of central concentration of recent star formation is lower, whereas fast-rotating Irrs tend to exhibit the highest central concentrations. The same trend also holds for the star formation activity: low-mass dIrrs with no measurable rotation also have lower star formation rates (Roye & Hunter 2000; Parodi & Binggeli 2003).
How does star formation progress in irregular galaxies? Irrs and the more massive dIrrs usually contain multiple distinct regions of concurrent star formation. These regions often remain active for several 100 Myr, are found throughout the main body of these galaxies (see above), and can migrate. This is illustrated in Figure 2 for the LMC, where the large-scale star formation history of the last ~ 250 Myr (approximately one rotation period) is shown (see Grebel & Brandner 1998 for full details). Note how some of the active regions have continued to form stars over extended periods and propagated slowly, whereas others only became active during the past 30 Myr. The star formation complexes resemble superassociations and may span areas of a few hundred pc (Grebel & Brandner 1998). In supershells, typical time scales for continuing star formation on length scales of 0.5 kpc range from 15 to 30 Myr, usually without showing clear signs of spatially directed propagation with time (see also Grebel & Chu 2000 and Section 3.1). CO shows a strong correlation with HII regions and young (< 10 Myr) clusters, but only little with older clusters and supernova remnants (Fukui et al. 1999; cf. Banas et al. 1997). Massive CO clouds have typical lifetimes of ~ 6 Myr and are dissipated within ~ 3 Myr after the formation of young clusters (Fukui et al. 1999; Yamaguchi et al. 2001). Spatially resolved star formation histories have also been derived for two dIrr galaxies just beyond the Local Group covering the past 500-700 Myr. They reveal similar long-lived, gradually migrating zones of star formation (Sextans A: Van Dyk, Puche, & Wong 1998; Dohm-Palmer et al. 2002; GR8: Dohm-Palmer et al. 1998), as seen in the more massive Magellanic Clouds.
Figure 2. Large-scale star formation patterns in the Large Magellanic Cloud spanning the past ~ 250 Myr. The individual dots correspond to age-dated Cepheids (upper panel) and supergiants (lower panel). A few prominent features like the LMC bar, supershell LMC4, and 30Doradus are marked by solid and dashed lines. Note how star formation migrated along the LMC's bar and finally vanished in its southernmost past, and how other regions such as 30Doradus and LMC4 only became strongly active over the past ~ 30 Myr. Within the time scales depicted here, which incidentally correspond to roughly one rotation period, stars are not expected to have migrated far from their birthplaces. (From Grebel & Brandner 1998.)
In low-mass dIrrs one usually observes only one single low-intensity star-forming region. DIrrs and transition-type dIrr/dSph galaxies tend to be fairly quiescent, often having experienced the bulk of their star formation at earlier times. (In fact, transition-type dwarfs resemble dSphs in their gradually declining star formation rates; see Grebel et al. 2003 for details.) Evidence for migrating star formation is found in low-mass dIrrs as well, albeit on smaller scales owing to the smaller sizes of these galaxies (e.g., Phoenix: Martínez-Delgado, Gallart, & Aparicio 1999).