3.2. More Distant Dwarf Irregular Galaxies
The other Local Group dIrrs are more distant from the dominant spirals, and fairly isolated. Interactions may still occur, but if this happens the interaction partners tend to be gas clouds rather than galaxies. Generally, star formation activity and gas content decrease with galaxy mass, but the detailed star formation histories and ISM properties of the dIrrs present a less homogeneous picture.
NGC6822, a dIrr at a distance of ~ 500 kpc, is embedded in an elongated HI cloud with numerous shells and holes. Its total HI mass is 1.1 × 108 M, ~ 7% of its total mass. The masses of individual CO clouds reach up to (1 - 2) × 105 M (Petitpas & Wilson 1998), while the estimated H2 content is 15% of the HI mass (Israel 1997), and the dust-to-gas mass ratio is ~ 1.4 × 10-4 (Israel, Bontekoe, & Kester 1996). NGC6822 contains many HII regions. Its huge supershell (2.0 × 1.4 kpc) was likely caused by the passage of and interaction with a nearby 107 M HI cloud and does not show signs of expansion (de Blok & Walter 2000). The older stars in IC10 describe an elliptical, extended halo (Letarte et al. 2002) distinct from the elongated HI distribution. The latter, however, is traced closely by a population of young blue stars (~ 180 Myr) that appear to have formed following the interaction with the passing HI cloud (de Blok & Walter 2003; Komiyama et al. 2003) some 300 Myr ago. In NGC6822, the HI distribution is thus only slightly more extended than the stellar loci.
The HI of IC10 (distance 660 kpc) is 7.2 times more extended than its Holmberg radius (Tomita, Ohta, & Saito 1993). While the inner part of the neutral hydrogen of IC10 is a regularly rotating disk full of shells and holes, the outer HI gas is counter-rotating (Wilcots & Miller 1998). IC10 is currently experiencing a massive starburst, which is possibly triggered and fueled by an infalling HI cloud (Saito et al. 1992; Wilcots & Miller 1998). IC10 contains a nonthermal superbubble that may be the result of several supernova explosions (Yang & Skillman 1993). The masses of the CO clouds in IC10 appear to be as high as up to 5 × 106 M (Petitpas & Wilson 1998), which would indicate that more than 20% of this galaxy's gas mass is molecular. Owing to the high radiation field and the destruction of small dust grains, the ratio of far-infrared [CII] to CO 1-0 emission is a factor 4 larger than in the Milky Way (Bolatto et al. 2000), resulting in small CO cores surrounded by large [CII]-emitting envelopes (Madden et al. 1997). Two H2O masers were detected in dense clouds in IC10, marking sites of massive star formation (Becker et al. 1993). The internal dust content of IC10 is high, and its properties prompted Richer et al. (2001) to suggest that this galaxy should actually be classified as a blue compact dwarf.
Less detailed information is available for the ISM in the other Local Group dIrrs, which do not appear to be involved in ongoing interactions and which are evolving fairly quiescently. The HI in these dIrrs may be up to 3 times more extended than the optical galaxy and is clumpy on scales of 100 to 300 pc. The most massive clumps reach ~ 106 M. HI concentrations tend to be close to HII regions. Some dIrrs contain cold HI clouds associated with molecular gas. The total HI masses are usually < 109 M, and less than 107 M for transition-type dwarfs. The center of the HI distribution coincides roughly with the optical center of the dIrrs, although the HI may show a central depression surrounded by an HI ring or arc (e.g., SagDIG, LeoA), possibly a consequence of star formation, or the HI may be off-centered (e.g., Phoenix; St-Germain et al. 1999). In low-mass dIrrs there are no signatures of rotation, but these may be obscured by expanding shells and bubbles. Further details are given in Lo, Sargent, & Young (1993), Young & Lo (1996, 1997), Elmegreen & Hunter (2000), and Young et al. (2003).
Lower gravitational pull and the lack of shear in the absence of differential rotation imply that HI shells may become larger and are long-lived (Hunter 1997). Diameters, ages, and expansion velocities of the HI shells increase with later Hubble type (Walter & Brinks 1999) and scale approximately with the square root of the galaxy luminosity (Elmegreen et al. 1996). Shell-like structures, HI holes, or off-centered gas may be driven by supernovae and winds from massive stars following recent star formation episodes or tidal interactions.
For a review on nebular abundances in Irrs, see the contribution by Garnett (2004). Here it should only be mentioned that the effective yields in Irrs computed from gas-phase abundances are lower than those in the main stellar disks of spirals. Lower effective yields are also correlated with lower rotational velocities (Garnett 2002). This is interpreted as preferential metal loss through winds in the more shallow potential wells of Irrs and dIrrs, but may also be due to lower astration levels (e.g., Pilyugin & Ferrini 2000). For a review of the general ISM properties in Local Group dwarf galaxies, see Grebel (2002a).