|Annu. Rev. Astron. Astrophys. 1991. 29:
Copyright © 1991 by Annual Reviews. All rights reserved
8.1 Star Formation Efficiencies
Several studies have investigated the star formation efficiency in interacting/merging galaxies (Sanders & Mirabel 1985, Young et al 1986a, b, Solomon & Sage 1988, Sanders et al 1991, Tinney et al 1990) and found that strongly interacting galaxies have mean LIR / M (H2) ratios 5-10 times higher than isolated galaxies. A similar enhancement is found in the LH / M (H2) ratios for interacting/merging galaxies versus isolated galaxies (Allen & Young 1989). Most of the studies have also indicated that only the most closely interacting systems have a significant enhancement. These results suggest that the perturbations due to galaxy-galaxy interactions result in more efficient production of massive stars.
Figure 11 shows a plot of the ratio LIR / M (H2) versus dust temperature for 150 galaxies, in which the points are coded by the galaxy's environment (dwarf irregulars, isolated, group/cluster, and interacting/merging galaxies). Although the most strongly interacting galaxies often have high star formation efficiencies, there is an order of magnitude scatter in the L/M observed among both the isolated and interacting galaxies. This scatter, which is considerably larger than the measurement uncertainties (± 30%), may reflect time evolution of the IR luminosity and/or the H2 content for the starbursts in the interacting galaxies (Young et al 1986a, b). Scalo (1987) has suggested that bursts of star formation control the evolution even in isolated galaxies, and this might account for the large scatter in the star formation efficiency for the normal galaxies.
Figure 11. Comparison of the ratio LIR / M (H2) with S60 / S100 for the galaxies in Figure 7. The galaxies are coded by environment is follows: filled circles for isolated galaxies, open squares for group and cluster galaxies, stars for interacting/merging galaxies, and open triangles for dwarf and irregular galaxies. The merging/interacting galaxies have high values of both LIR / M (H2) and S60 / S100 relative to the isolated galaxies.
Several mechanisms for the enhancement in the yield of young stars per unit mass of molecular gas in interacting/merging galaxies have been suggested. On one hand, the galaxy-galaxy interaction may cause a new star formation mechanism to operate such that more stars form per unit molecular mass. Alternatively, the physical process that causes high mass stars to form in merging/interacting galaxies may be no different than that in isolated galaxies, but the interaction may enhance its effectiveness. We favor the latter explanation, i.e. the interaction causes a star formation mechanism already present to operate at an increased level, as opposed to invoking a new mechanism.
One process that probably becomes important during galaxy-galaxy interactions is that of cloud-cloud collisions. Noguchi & Ishibashi (1986) and Olsen & Kwan (1990) have performed numerical simulations of galaxy-galaxy interactions including gas clouds as well as stars. They find that the cloud-cloud collision rate increases as bridges and tails develop during violent galaxy-galaxy encounters. In the specific cases treated by Noguchi & Ishibashi, the cloud collision rate is elevated by an order of magnitude at 3 x 108 years after the closest approach of the perturber. If cloud-cloud collisions are responsible for high mass star formation galaxies (Scoville et al 1986a), the enhanced rate of cloud-cloud collisions in interacting galaxies should result in an increase in the star formation rate per unit H2 mass.