4.1 Dwarf Irregular Galaxies
Dwarf irregulars are in general well described by exponential surface brightness profiles. They contain fair amounts of neutral and perhaps molecular gas and in general show evidence for star formation, at low (or moderate) rates. There are some examples of galaxies of intermediate type between dI and dE, e.g. the Local Group dwarfs Pegasus and Phoenix. Such galaxies have no signs of ongoing star formation, but contain gas (Mateo 1998). Dwarf irregulars partially overlap blue compact galaxies (BCGs) in the classification criteria, i.e. many BCGs are indeed irregular, and dIs often contain bright H II regions which may be picked up in emission line surveys. Of course the transition is gradual and to some extent arbitrary. There is also some overlap with the LSBG class of galaxies discussed in Sect. 4.3.
The Local Group contains around a dozen (depending on the magnitude limit) dIs most of which are rather metal-poor. Dwarf irregulars are also found in the local field and in nearby clusters and groups, but metallicity determinations become rarer with increasing distance.
Dwarf irregulars in general show ongoing star formation and H II regions. The star formation history (SFH) of Local Group dIs has been reviewed recently by Grebel (1998), and Mateo (1998). Most dIs seem to have experienced more or less continuous star formation over a Hubble time. Many Local Group dIs show evidence for an episodic star formation history to a varying degree (e.g. Leo A, Tolstoy et al. 1998), but it is difficult to say if the past SFH should be described in terms of short bursts or more extended periods of increased SF as compared to the time averaged value. There seems to be no clear pattern when comparing different galaxies, i.e. no support for co-ordinated bursts. Star formation in dIs is probably governed by different mechanisms than in spiral galaxies, i.e. there are no density waves available for triggering star formation. Despite their closeness, it is quite fair to say that the regulation of star formation in the Local Group dIs (and dEs too) is not yet well understood. Interactions, ram-pressure stripping, superwinds and other recipes have been suggested as processes governing the star formation history and morphological segregation, but results are still inconclusive.
Young and Lo (1996, 1997b) find that neutral hydrogen in dIs exists in two phases and that the presence of cold H I may be the key factor by which a dI is presently forming stars or not. Intermediate dE/dI types do in some cases have gas, but not in the form of cold H I, which could explain the lack of present star formation (Young and Lo 1997b). The H I content of dIs in the nearby Universe has been investigated by e.g. Thuan and Seitzer (1979), Schneider et al. (1992, 1998) and Côté et al. (1997); see Skillman (1996) for a review. The dynamics of dIs can be investigated through H I rotation curves, and dIs (as most other dwarfs) turn out to be dominated by dark matter (cf. Mateo 1998).