As discussed in the introduction, there are differences between irregulars and spirals in terms of their interstellar mediums. In particular there is the decreased dustiness and abundances of heavy elements, and the low rotation velocities and near rigid-body rotation curves that mean reduced levels of shear and density shock waves in irregular galaxies. All of these differences could, in principle, translate into differences in the star formation process (see, for example, Shields & Tinsley 1976, Wolfire & Cassinelli 1987, Larson 1983).
One major difference is that irregular galaxies have less shear in their
ISMs than spirals. According to Larson (1983), shear contributes to
the growth of perturbations, and therefore one might expect irregulars
to form stars more slowly. On the other hand, Larson also points out
that less shear and thicker disks should allow the growth of larger gas clouds.
Kennicutt, Edgar, & Hodge (1989)
have counted the numbers of H II regions as a function of the H
luminosity in a variety of spiral
and irregular galaxies. They found that Im galaxies have statistically
more giant H II regions than do Sb or Sc spirals which is explained
by Elmegreen et al.
(1996) as due to a shorter formation time within
young star-forming complexes in small galaxies.
For example, the irregular NGC 2366, which has a star formation rate
per unit area that is comparable to that of the LMC, contains a giant H
II region that is almost twice the H luminosity of 30 Doradus, the dominant giant H II region in
the LMC. The giant H II region in NGC 2366 accounts for
over two-thirds
of the total H luminosity
of that galaxy. The 30 Doradus nebula also demonstrates that at least
some of these regions are associated with enormous complexes of neutral
gas as well (Luks & Rohlfs
1992, Cohen et al.
1988). The gas complex that hosts 30 Doradus contains about 40% of
the total molecular gas of the LMC as measured in CO
(Cohen et al. 1988),
equivalent to 20-100 typical Milky Way giant molecular clouds.
It also contains almost 3% of the total H I in the galaxy
(McGee & Milton 1966).
The presence of these giant H II regions means that large concentrations of massive stars have formed, and by concentrating the massive stars together in space and time, the disruption to the ISM by the stars will be greater than if they had formed in smaller star-forming units. Hence, the disproportionate number of giant H II regions in irregular galaxies should mean more disruption of the ISM. In addition, these giant gas clouds may be what is needed to form large, compact star clusters exemplified by R136 in 30 Doradus (Larson 1993). According to Elmegreen (1995b), the formation of these globular-sized clusters requires not only a large amount of gas but also a quick collapse into stars. According to Fleck (1980), shear contributes to turbulent flows that help stabilize clouds against collapse. The consequence to irregulars then could be that giant gas clouds are less stable and that they can collapse more quickly. Thus, irregular galaxies may be more able to form globular-like clusters today than other non-interacting disk galaxies, and in fact irregulars do seem to host many of the young super star clusters found outside of merging systems. The LMC is also well known for its numerous ``populous'' clusters, star clusters that are less massive and compact than a typical globular cluster yet much more so than a typical OB association or open cluster.
Another difference between irregulars and spirals is the reduced
dust and metallicity. Models for the formation of massive stars show
that radiative forces on dust play a role in halting accretion flows
(Larson & Starrfield 1971,
Wolfire & Cassinelli 1987).
Thus, to make stars with masses > 60 M
, one needs a reduced
dust-to-gas ratio.
This implies that the upper stellar mass limit in irregulars should
be higher than that in the Milky Way, for example. However,
observations do not confirm this. In fact, no variations in the
upper stellar mass limits are seen between the Milky Way and
the Magellanic Clouds (Massey et
al. 1995a), suggesting the need
for alternate models for the formation of massive stars.
Figure 11. Slopes of IMFs measured for OB associations and clusters and for massive field stars and small Galactic H II regions. The dashed line is the Salpeter (1955) slope for the solar neighborhood. See Hunter (1995) and Hunter et al. (1997b) for references. |
Furthermore, the stellar IMF's for intermediate and massive stars in the Magellanic Clouds and nearby spirals are quite similar. The slopes of IMFs that have been measured from star counts in these galaxies are summarized graphically in Figure 11 where the IMF slope is compared to that first determined for the solar neighborhood by Salpeter (1955). Although there does seem to be a difference in the massive star contents between small star-forming events and larger ones (Larson 1985, Massey et al. 1995b), there does not seem to be a difference among galaxies. Thus, the stellar products, as a diagnostic of the star formation process, suggest that the local star formation process in irregulars is not different from that in spirals today in spite of the different environmental parameters within the galaxies (but see Larson 1991 for a discussion of IMFs in the early life of spirals). In addition, so far as one can measure it, star formation efficiencies in irregulars seem to be comparable to those measured for spirals; the percentage of the mass of clouds that actually ends up in stars is a few percent in most normal galaxies (Thronson & Telesco 1986, Thronson et al. 1988).
In summary then, the national forest service fire look-out mentioned in Section 2 was right: The local star formation process (that is, what happens inside a gas cloud once it forms) is the same in irregulars as it is in spirals today in spite of differences in the ISM. However, irregulars have a disproportionate number of giant H II regions which in turn means more disruption of the ISM.