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

4.2 Bursts of Star Formation

In spite of the previous section's tone, it is clear that a constant stellar production rate does not fit the evolution of all Irrs. Repeatedly in the literature, people find themselves forced to conclude that the current global astration rate in a system is much higher than it used to be. Bursts have been suggested for various high surface brightness Irrs from a comparison of evolutionary models of stellar populations with observed broadband colors for late-type galaxies (15, 220, 313, 333, 349), from colors and emission-line strengths in Markarian galaxies (177, 178), from population studies for the Magellanic Clouds (8, 43, 122, 168) and M82 (257), from metallicity enrichment rate arguments (4, 225, 226, 258, 361), from radio observations of Markarian galaxies (33, 148a), and from star cluster studies of NGC 5253 (370), to name a few examples. Similarly, amorphous galaxies such as NGC 1705, which appear to be involved in OB star formation at high rates over most of their optical dimensions, are likely to be in burst phases (22, 219), as are clumpy Irrs (66). It is also evident from the prevalence of very blue galaxies in binary systems that interactions affect star formation processes and may stimulate bursts (10, 11, 31, 201, 220).

It is not immediately obvious, however, how seriously one should take the evidence for star formation bursts as a general evolutionary feature of noninteracting Irrs. The formation of OB stars, after all, occurs via gravitational collapse in interstellar cloud complexes, a process that produces spatially compact OB associations and star clusters. Thus, as Searle et al. (313) explicitly recognized, star formation is an intrinsically grainy process, and in small galaxies the normal evolution should proceed as a series of ``bursts'' associated with the appearance and decay of individual star-forming complexes (cf. 86). These statistical effects will be most important in small galaxies, since the blue luminosities of single star-forming complexes probably do not much exceed M_B ~ - 15 (388) and spatial sizes of ~ 1 kpc (170, 188). Star formation bursts in small galaxies therefore do not necessarily indicate any evolutionary anomalies. There are also some difficulties in interpreting the empirical evidence for bursts in any galaxies: With metallicity enrichment arguments, one can raise the questions of whether the system is closed and what volumes of gas must be considered (see previous discussions); and with colors, one can say that the evolutionary path to any set of optical colors is not unique. Despite these problems, it is still clear that even some nearby, noninteracting Irrs cannot be explained without global star formation bursts [e.g. NGC 2915 (320), Haro 22 (126)]. In NGC 1569, for example, OB stars are spread over an area of many kpc (84, 188, 191), the current star formation rate is 10 times higher than its average past rate (126), and the optical luminosity is more than 10 times the single event maximum. Alternatively, if galaxies like NGC 1569 are not bursting, then they must be peculiar in other ways, i.e. they must be young or have an unusual IMF that favors production of OB stars.

The existence of Irrs currently undergoing global bursts of star formation implies that (a) some mechanism must exist to organize star formation on large scales, and (b) there must be Irrs of the same basic types as bursters that are not now active. In fact, we should see systems in all phases of postburst decays. The low surface brightness, low star formation rate dwarf Irrs come immediately to mind as postburst candidates. These systems may have had higher star formation rates in the past, but spectrophotometric data indicate that the metallicities and the stellar populations are not consistent with the picture of their being the low star formation rate states of high surface brightness Irr galaxies (189).

High and low surface brightness Irrs, in fact, seem to have experienced parallel recent evolutionary histories that have produced similar integral properties, such as gas metallicities and stellar population mixes. These galaxies thus primarily differ as a result of stellar surface density, but it is also noteworthy that lower surface brightness Irrs rarely contain luminous star-forming complexes (and when exceptions occur, as in NGC 2366, they are obvious but do not change the overall surface brightness). Perhaps factors such as gas density prevent the low surface brightness dwarfs from forming gas clouds in the size and quantity necessary for large star-forming complexes, which are typical of high surface brightness Irrs. In NGC 6822, for example, many HII regions are small (211), and only a few HII regions require more than a single O star for their ionization (137). Based on the Hodge (170) study of HII region sizes in Irrs, this situation seems to be typical. Star formation in dwarfs evidently proceeds in an unspectacular manner due to the lack of giant star-forming complexes. This results in low surface brightness dwarfs having followed approximately the same evolutionary paths as giant Irrs, in agreement with the observations, but leaves the issue of descendants of global burst Irrs unresolved.