Annu. Rev. Astron. Astrophys. 1984. 22: 37-74
Copyright © 1984 by . All rights reserved

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Irregular galaxies (Irrs) usually are smaller, less massive, and optically dimmer than commonly studied giant spirals, S0s, and classical ellipticals. At first glance they also appear to be rare objects that make up only a few percent of the major bright galaxy catalogs (87, 300). Is there then much reward in pursuing such faint and elusive galaxies? The answer to this question turns out to be a surprisingly strong ``yes.'' An examination of more nearly complete samples of galaxies (213, 341, 366) reveals that Irrs account for a substantial (1/3-1/2) fraction of all galaxies, and that they certainly are dominant by number density among actively star-forming galaxies. A diversity of nearby examples therefore abound [including, of course, the Large and Small Magellanic Clouds (LMC and SMC)], making Irrs prime targets for detailed investigations of galactic stellar content and star formation processes. Recognition of their structural simplicity has provided an additional stimulus for recent interest in Irrs as tests for theories of galaxy structure and evolution. These galaxies also are comparatively unevolved, and thus they may yield further rewards by allowing conditions to be defined in the poorly understood realm of low-density extragalactic systems, which retain considerable information about galaxy formation (101, 327).

In the broadest sense, the irregular galaxy class is loosely defined (e.g. see illustrations in 9, 23, 378, 379). Hubble (175, 176) originally built on earlier nebular classification schemes (e.g. 153) and considered galaxies to be ``irregular'' if they showed chaotic, nonsymmetrical blue-light distributions, in contrast with the axial symmetry of normal ``regular'' systems. Later classification systems subdivide irregular galaxies into two major groups: Magellanic systems (Irr I, Im), which resemble the Magellanic Clouds; and peculiar, often amorphous galaxies, which are classified as Irr II or I0 systems (72, 174, 290, 292).

Unfortunately, irregular structures may arise from a variety of physical causes, and as a result a wide range of physical types of irregular galaxies are known to exist: (a) There may be substantial chaos in the projected stellar mass distributions in galaxies, although such nonequilibrium structures are unlikely to survive for more than a few rotation periods (~ 109 yr). Most currently known galaxies in disturbed states seem to be involved in galaxy-galaxy interactions (311, 358, 359, 384, 392, 393), although newly formed galaxies could also find themselves in this situation (45). Some galaxies classified as Irr II or I0 also belong in the category of interacting galaxies. (b) Similarly, unusual distributions of dense interstellar gas across the face of a galaxy may produce an optical image that is mottled by dark lanes. The dusty Irr II galaxies discussed by Krienke & Hodge (214) belong in this group. As these authors note, extended interstellar matter also can lead to emission filaments and reflection nebulae on galactic scales that give rise to abnormal optical morphologies (e.g. M82; 34, 67, 281, 332). The origins of peculiar global distributions of interstellar matter are not well understood, but some cases are caused by interactions between galaxies (65, 136, 373). (c) Young, massive stars have very low mass-to-light ratios, and thus sites of recent star formation stand out against even moderately high density projected stellar backgrounds. Furthermore, OB stars tend to form in spatially localized groups that are seen as OB associations or perhaps larger units with dimensions of up to 1 kpc (``constellations,'' 253; ``star complexes,'' 91). Thus, in galaxies with low background stellar density levels and spatially incoherent star formation patterns, patches of young stars stand out against symmetrically distributed older stars and give rise to irregular optical brightness structures. This effect is beautifully illustrated in UV photographs of the Large Magellanic Cloud that were obtained from the lunar surface during the Apollo program (263, 264).

Most irregular galaxies belong in the third category, which includes Magellanic-type irregulars and spirals (70, 72, 78, 233, 290, 292, 365, 366, 368) as well as a smattering of genetically related systems such as intergalactic H II regions (304, 312), amorphous galaxies (defined by 294, 300: blue galaxies with relatively smooth optical light distributions; a subclass of Irr IIs or I0s: 174, 321; also ``blue Es'': 104), or luminous, clumpy Irr galaxies (50, 51, 149, 150, 302). Hereafter, we limit our scope to the Magellanic galaxy family (and mainly systems that are not involved in obvious galaxy-galaxy interactions), which we refer to simply as Irrs.

The Irrs blend from lower luminosities smoothly into the spirals, as emphasized by both G. de Vaucouleurs and A. Sandage, and thus can be viewed as an extension of actively star-forming disk galaxies to lower densities and luminosities. Luminous members of the Irr family overlap with spirals in optical luminosity and are often preferentially selected in surveys for blue or emission-line galaxies (e.g. the G. Haro and B. Markarian surveys; see 208). Thus Irrs probably now comprise the bulk of known galaxies with dominant hot stellar populations. We close this section with Figure 1, which illustrates NGC 4449, a classical nearby giant Irr. In the remainder of this article, we discuss specific physical characteristics of Irrs that are structurally related to NGC 4449, with an emphasis on exploiting the Irrs as probes of evolutionary processes in galaxies.

Figure 1

Figure 1. Blue photograph of the nearby (D ~ 5 Mpc), giant irregular galaxy NGC 4449, taken for the authors by G. Lelievre with the prime-focus camera on the Canada-France-Hawaii 3.6-m telescope. The scale of this print is approximately 3" mm-1; north is to the upper right corner, and east is counterclockwise. The chaotic structure of this galaxy largely results from many bright star-forming complexes superimposed on a strongly barred, amorphous, older stellar background. A few dark nebulae (light regions on this negative copy) are also visible.

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