The remaining Local Group dwarf irregulars (dIrrs) are more distant from the dominant spirals, and fairly isolated. While their star formation activity and gas content generally decrease with decreasing galaxy mass, their star formation histories and ISM properties present a less homogeneous picture when considered in detail. We first present the contrasting examples of two comparatively high-mass dIrrs and then move on to the low mass end.
The HI of IC10 (distance 660 kpc) is 7.2 times more extended than its Holmberg radius . While the inner part is a regularly rotating disk full of shells and holes, the outer HI gas is counterrotating . IC10 is currently undergoing a massive starburst, which may be triggered and fueled by an infalling HI cloud (, ). Only upper limits have been established for the diffuse X-ray emission , which may be due to the high foreground absorption toward IC10. A non-thermal superbubble was detected that may be the result of several supernova explosions . The masses of the molecular clouds in IC10 are as high as 0.3 to 5 . 106 M . 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 , resulting in small CO cores are surrounded by large [CII]-emitting envelopes . Two H2O masers were detected in dense clouds in IC10, marking sites of massive star formation . The internal dust content of IC10 is high .
NGC6822, a dIrr at a distance of ~ 500 kpc, is also embedded in an elongated HI cloud with numerous shells and holes that is much more extended than its stellar body . Its total HI mass is 1.1 . 108 M, ~ 7% of its total mass. The masses of individual CO clouds reach up to 1 to 2 . 105 M , while the estimated H2 content is 15% of the HI mass , and the dust to gas mass ratio is ~ 1.4 . 10-4 . In comparison to IC10, NGC6822 is fairly quiescent, although it 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 .
The HI in low-mass dIrrs may be up to three 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, while dIrrs without cold HI also do not show ongoing star formation. The total HI masses are usually < 108 M. 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). In contrast to the more massive dIrrs, the low-mass dIrrs show little to no rotation and appear to be dominated by chaotic motions. Details are given in , , , .