|Annu. Rev. Astron. Astrophys. 1984. 22:
Copyright © 1984 by . All rights reserved
3.3 Star-Forming Complexes and the Interstellar Medium
The great star-forming complexes found in Irrs are very similar to those associated with giant HII regions in spirals, as in M33 or M101 (188, 189, 377). This includes their optical properties (225), ultraviolet spectra (180), sizes (188, 203, 204, 297, 298, 299), kinematics (compare 188 with 285), and morphology (Hunter & Gallagher, in preparation). These similarities indicate that once a gas cloud complex is stimulated or naturally reaches a stage where stars will condense, the region forgets what kind of galaxy it is in, and that the upper limit to star-forming cloud complex sizes is similar in spirals and Irrs. In addition, Hodge (169) has demonstrated that the distribution of sizes of all HII regions in a galaxy can be fit by an exponential law, although this fit steepens for less luminous parent galaxies. The giant HII regions in some Irrs are then found to be anomalously large relative to this distribution, and therefore they could result from special (but common) cloud formation processes (98).
There are also some differences between the interstellar mediums of Irrs and spirals. First, the Irrs appear to be deficient in large, dense interstellar clouds as judged from the low optical visibility of dark clouds, although a few dark nebulae are clearly present, especially on smaller spatial scales (161, 163, 165, 166, 188). Star-forming regions in the Galaxy and in spirals are most often adjacent to dusty areas that mark molecular clouds (97), so a dearth of dark clouds in star-forming galaxies is unexpected. An examination of blue and red passband images. Balmer decrements, and optical- and radio-determined star formation rates shows that even in and around star-forming sites, optical interstellar extinction in Irrs tends to be low [maximum E (B - V) 0.5 (188, 191, 340); for the Magellanic Clouds, see, for example, 89, 111, 184, 186, 234, 271]. This may be in part a reflection of the underabundance of heavy elements, which certainly results in some modifications of dust properties (e.g. 283a, 306). In addition the low surface brightnesses and irregular light distributions could hinder the detection of dust in Irrs. A lack of extensive regions of high column density gas, however, is probably the main factor. The notable exception among Irrs is the peculiar galaxy M82, which is loaded with dust even as compared with spirals, although M82 is also relatively metal rich (257). Why some galaxies have optically obvious dust clouds and others do not and how the presence or lack of dark clouds affects the star formation processes are not known, since high and low star formation rate examples of both extremes are known to exist.
Furthermore, the high surface brightness Irrs are underluminous in CO molecular microwave emission relative to regions in spirals with similar luminosities and stellar content (95, 135a, 390). Although the global quantity of molecular matter evidently is down, the presence of vigorous star-forming regions in Irrs argues that on local scales the molecular clouds are normal. It is possible that the current star formation activity has disrupted most of the parent clouds, but then we should find systems with a reduced star formation rate that are full of CO clouds. An alternative is for diffuse molecular clouds to be comparatively short lived in Irrs, which is possibly related to the low specific angular momentum of Irr disks. If molecular clouds formed in low angular momentum environments in fact collapse more readily than in spirals, then in a local sense Irrs may be more efficient star-formers than spirals, despite the absence of dynamical forcing by arms. This could explain the anomalous presence of spectacular star-forming complexes (e.g. giant HII regions) in otherwise undistinguished galaxies (but see 35). Refueling from outer gaseous reservoirs (disks, halos; 126) or differences in the thermal structure of the interstellar medium are also important factors in determining the state of interstellar matter that potentially could differentiate Irrs from spirals.