Annu. Rev. Astron. Astrophys. 1982. 20: 517-45 Copyright © 1982 by Annual Reviews. All rights reserved

5.3. Implications for Star Formation

On the basis of comparisons between H₂ and HI surface densities and radio and infrared surface brightnesses, one can construct models for massive star formation on a galaxy-wide scale. We briefly mention some attempts at this.

Talbot (1980) used reported values of the radio continuum surface brightness and of the surface densities of HI and H₂ to find the rate of massive star formation per unit mass of gas in our Galaxy and in M83. He found this rate to peak sharply in both galaxies where the rotation frequency is 65 km s^-1 kpc^-1 (at 2-4 kpc galactocentric radius), and farther out to be proportional to the difference between the rotation and spiral pattern frequencies. In both galaxies. the rate of formation of massive stars is more significantly correlated with the surface density of H₂ than with that of HI or of the total gas. Talbot argues that the distribution of low mass stars is not obviously related to the density-wave theory, unless there has been a substantial inward radial flow of gas; therefore his results appear to require two modes of star formation with different initial mass functions.

Young & Scoville (1981) come to similar conclusions about bimodal star formation. In matching the CO distributions to the exponential distributions of the blue luminosity in NGC 6946 and IC 342, they connect the molecular clouds with the moderately young stars (age < 2 x 10⁹ yr). The absence of CO counterparts to the spiral pattern then suggests that the very massive stars defining the arms are formed with a different IMF. Furthermore, the direct proportionality of blue luminosity and CO emission, together with the authors estimate that H₂ dominates the interstellar mass over the entire bright disks of these galaxies, imply that the star formation rate must be proportional to the first power of the inferred gas density.

In the ballistic-cloud model (Bash & Peters 1976, Bash et al. 1977, Bash 1979), dense clouds form in the spiral pattern at the post-shock velocity and move ballistically for a mean lifetime of 4 x 10⁷ yr. Massive stars, manifested as OB associations, are typically formed 2.5 x 10⁷ yr after cloud formation; the remainder of the cloud lifetime is presumably the time required for these stars to dissipate or fragment their parent cloud. Bash (1979) shows that this model will accurately reproduce observed optical surface brightnesses and colors for galaxies like M51. Recently, Bash & Visser (1981) applied these considerations in the construction of a model for star formation in M81. Their predicted molecular cloud distribution is consistent with the single CO detection in its disk (Combes et al. 1977a).