Irr and dIrr galaxies are usually gas-rich galaxies with ongoing or recent star formation. They are preferentially found in the outer regions of groups and clusters as well as in the field. Irrs and massive dIrrs exhibit solid body rotation, while low-mass dIrrs seem to be dominated by random motions. Spiral density waves are absent.
Irrs and dIrrs are often embedded in extended HI halos, which, in the absence of interactions, appear fairly regular. In low-mass dIrrs, the centroid of the HI distribution does not necessarily coincide with the optical center of the galaxy, and occasionally annular structures are seen. The neutral gas tends to be flocculent, dominated by shells and bubbles, and driven by the turbulent energy input from massive stars and supernovae. Molecular gas and dust form less easily and are more easily dissociated due to the high UV radiation field and fewer coolants in low-metallicity environments.
Irrs and dIrrs usually contain multiple distinct zones of concurrent star formation. Extended regions of active star formation tend to be long-lived and gradually migrate on time scales on a few tens to hundreds of Myr. Stochastic self-propagating star formation seems to be the main driver of star formation activity. There is no need for external triggering. In quiescently evolving dIrrs and/or dIrrs with slow or no rotation (usually the less massive dIrrs), the degree of central concentration of star formation is small, while the reverse trend is true for more massive and faster rotators. The formation of populous clusters seems to be preferred in more massive and/or interacting dIrrs. Generally, gas consumption is sufficiently low that star formation in Irrs and dIrrs may continue for another Hubble time. On global scales the star formation rate of Irrs and dIrrs is close to constant, with amplitude variations of factors of 2-3.
Old stellar populations are ubiquitous in all Irrs and dIrrs studied in detail so far, although their fractions vary widely. In contrast to the many scattered young OB associations and superassociations, older populations show a smooth and regular distribution that is much more extended than that of the young populations. Both young stellar populations and HII regions agree very well in their abundances, underlining the chemical homogeneity of Irrs and dIrrs. However, taken at face value, intermediate-age and old populations tend to exhibit considerably more scatter in their metallicity. There are indications that star clusters of the same age may differ by several tenths of dex in metallicity, although observational biases cannot yet be fully ruled out. Overall, Irrs and dIrrs follow the expected trend of increasing metal enrichment toward younger ages; the currently available data do not yet permit one to unambiguously distinguish between infall and leaky-box versus closed-box chemical evolution, nor to reliably evaluate the importance and impact of possible bursts.
Substantial
progress is being made not only in spectroscopic measurements of stellar
metallicities, but also in the determination of individual element
abundances. The [
/ Fe]
ratios in Irrs and dIrrs, which tend to
be lower than the solar ratio, and the lower effective yields
may be interpreted as indicative of lower
astration rates and a reduced contribution of Type II supernovae.
Other interpretations (different initial mass functions, leaky-box chemical
evolution with metal loss through selective winds) are being entertained
as well.
Correlations between gas content and distance from massive galaxies as well as morphological segregation indicate that environment (in particular gas loss through ram pressure or tidal stripping; see also Parodi, Barazza, & Binggeli 2002; Lee, McCall, & Richer 2003 for the Virgo cluster) does have an impact on the evolution of Irrs and dIrrs. Irrs and dIrrs follow the well-known relation of increasing mean metallicity with increasing galaxy luminosity. The offset in this relation from the relation for dSphs, such that dIrrs are more luminous than dSphs at the same metallicity, indicates that the early chemical evolution in these two galaxy types proceeded differently, with dSphs becoming enriched more quickly.
Acknowledgements
I would like to thank the organizers, particularly Andy McWilliam, for their kind invitation to this very interesting Symposium, and for their patience while my paper was finished. I am grateful to Jay Gallagher for a critical reading of the text.