|Annu. Rev. Astron. Astrophys. 1984. 22:
Copyright © 1984 by . All rights reserved
Irrs exist over an extreme range in optical luminosity extending from clumpy irregulars with MB -20 to intrinsically faint dwarfs with MB > - 13. Usually, luminous systems have moderate-to-high optical surface brightnesses (blue SB ~ 100 L pc-2 in an effective radius), while most dwarfs (i.e. galaxies with MB > - 16) are barely detectable above the night-sky background (blue SB ~ 10 L pc-2). There are exceptions: for example, dwarf ``blue compacts'' can have high surface brightness (361), and luminous examples of low-surface-brightness Irrs also exist (231, 352). Most Irrs, however, are low-surface-brightness dwarfs, which accounts for their rarity in catalogs of bright galaxies.
Optical surface photometry is available for a number of Irrs, most of which are dwarfs (e.g. 2, 3, 32, 71, 73, 74, 79, 160, 165, 166, 323). The mean radial brightness profiles of these galaxies are well represented by exponential intensity distributions I(R) = I(0) exp(-R) with scale lengths -1 ~ 1-3 kpc. The light distributions in Irrs are therefore similar in form, but of lower surface brightness and shorter scale length, to those of spiral galaxy disks (117, 310). We thus infer that Irrs have lower mean projected stellar densities than typical spirals. The smooth profiles of Irrs are often disturbed by (a) star formation activity, which produces islands of high surface brightness, especially in the inner regions; (b) by bars, which are common in Irrs; and (c) by the effects of individual luminous stars in nearer systems (79). Deep images (172, 199) sometimes reveal that the actively star-forming cores of Irrs are embedded in smooth halos that qualitatively resemble diffuse dwarf elliptical galaxies. This has led several authors to suggest a close structural relationship between elliptical and Irr dwarfs (39, 125, 229, 386).
In terms of integrated optical colors, the Irrs are the bluest of the ``normal'' galaxy classes, with (B - V) ~ 0.4 and (U - B) ~ -0.3 (77, 177, 178). Standard models of stellar populations can yield such blue colors for galaxies of normal cosmological age only with some difficulty which has fostered the idea that many Irrs have been detected in a stage of heightened star formation activity, i.e. very blue galaxies may be caused by star formation bursts (177, 178, 313). It also is possible that the colors could arise if the initial mass function (IMF) differed significantly from that deduced for stars in the Milky Way. Among intrinsically brighter Irrs, there is no strong trend between color and luminosity, but the fainter dwarfs in the David Dunlap Observatory (DDO) catalog of low surface brightness galaxies (365, 366) are systematically blue (82). This may be indicative of preferential selection of galaxies currently experiencing major star-forming events in samples of intrinsically small, faint galaxies. An example of the importance of star formation on the detectability of dwarf Irrs can be found by comparing VII Zw 403, a high surface brightness Local Group dwarf (361), with the low surface brightness Local Group Irr LSG 3 (363); the differences between these two galaxies stem from the presence of a small OB stellar complex in VII Zw 403.
Unlike spirals, Irrs often show a central bluing of optical color (81, 83). This implies a concentration of star-forming activity to the inner regions of the galaxy, which is consistent with observed steep radial falloffs in distributions of H II regions, supergiant stars, and other Pop I star indicators in the LMC (171) and other Irrs (165, 166, 188, 189, 291). Many years ago, de Vaucouleurs (72) remarked on the smooth transitions from the pure irregulars to true spiral galaxies. This in part involves a change in styles of OB star formation. Unlike Irrs, relative star formation rates in spirals are highest (and colors bluest) in the mid-to-outer optical disk. Thus, in spirals the young stellar component often has a flatter radial distribution than the overall light. The Sdm-Sm galaxies (the morphological transition between spiral galaxies and pure Magellanic irregulars) are intermediate cases in this regard, having extensive OB stellar components that extend well beyond the obvious older stellar amorphous backgrounds. This phenomenon does not seem to correlate with the form of the outer low-density H I distribution: for example, NGC 4449 has a very extensive H I envelope (376) but tight OB star distribution, while in NGC 4214 the H I profile is of more normal dimensions (7), even though far-flung OB stars abound.
Irr galaxies are only beginning to be extensively studied in the non-traditional X-ray, rocket ultraviolet, and infrared spectral regions, but interesting results have already appeared. Irrs may have high X-ray to optical flux ratios compared with nonactive spiral galaxies. This is consistent with the presence of binary X-ray sources in their large, young stellar population fractions (99, 100, 126, 333). Early OAO rocket-UV photometry revealed that Irr galaxies have integrated ultraviolet energy distributions somewhat like late B stars (59), a result that now has received support from a variety of UV observations (48, 49, 61, 239, 263). These energy distributions underscore the high-visibility, dominant role played by luminous OB stars in Irr galaxies, but they also clearly show the composite nature of the stellar populations in Irrs, since m1550 - V ~ 1.5 is redder than B stars.
Infrared JHK photometry of Irrs has been obtained by Aaronson (1), Thuan (349), and Hunter & Gallagher (in preparation). Most Irrs fall near globular clusters and star-burst nuclei in their JHK colors, and thus they contain cool, presumably evolved stars. Interpretation of these IR data is complicated by the important role of asymptotic giant branch stars in stellar populations of intermediate age and by uncertainties in red supergiant populations (193, 272, 279), but typical Irrs have IR colors near those expected on the basis of constant star formation rates (335; B. Tinsley, private communication, 1978). As emphasized by Thuan (349) and Huchra et al. (179), the blue V - K colors ( 1) of some galaxies, however, could indicate a very large proportion of young stars or large-amplitude star formation bursts. Problems here include the sensitivity of young red star populations to low metallicities (24, 44) and the need to observe the entire old stellar component, which may be distributed across an underlying galaxy of low surface brightness and of larger angular size than the prominent young stellar complexes.
IR photometry is thus a potentially powerful tool in understanding recent star formation histories of galaxies (122, 123). Longer wavelength, thermal infrared emission from dust in Irrs has been detected in only a very few cases, but since large infrared luminosities (at wavelengths of > 10 µm and especially ~ 100 µm) are characteristic of high star formation rates in a variety of galactic environments, we can expect Irrs to begin to show up in this category as improvements are made in far-IR sensitivity (282, 345, 382). By providing information on IR luminosities and spectral characteristics as functions of readily determined gas metallicities and star formation rates, the Irrs play an important role in calibrating models for thermal IR emission from dust in ``young,'' metal-poor galaxies.