Annu. Rev. Astron. Astrophys. 1991. 29: 581-625 Copyright © 1991 by Annual Reviews. All rights reserved

7.1 The Star Formation Efficiency in the Disks of NGC 6946 and M51

A complementary approach to understanding galaxy evolution is to compare the detailed distributions of the star formation tracers within the disks of individual galaxies. From a comparison of the CO distribution with past (blue light) and present (H or IR) tracers of star formation, one can qualitatively infer the history of the star formation. NGC 6946 (Scd) is a nearby face-on spiral galaxy that is ideal for comparing the distributions of young stars and gas within both the disks and the spiral arms. It is also isolated, with no significant companions within 1 Mpc.

In NGC 6946, the azimuthally averaged distributions - FIR, blue light, H, radio continuum, and CO (see Tacconi & Young 1986 for references) - show the same radial fall-off (Figure 9), which is unlike that of the atomic gas. If the blue light measures the star formation rate integrated over the last ~ 2 x 10⁹ years (cf. Searle et al 1973, Gallagher et al 1984, Sandage 1986), and the H flux measures the current rate of formation of high mass stars, the fact that the blue / CO and H / CO ratios are constant as a function of radius indicates that both the present-day formation rate for high mass stars and the long-term integrated formation rate for intermediate mass stars are proportional to the available supply of molecular gas; i.e. the star formation efficiency is approximately constant.

Figure 9. Comparison of the radial distributions of CO (H2), HI, H, blue and radio continuum in NGC 6946 from Tacconi & Young (1986). References are given in Tacconi & Young (1986).

In NGC 6946 (and most other luminous spiral galaxies), the H₂ and HI radial distributions are entirely different. If the ratio of H₂ / HI surface densities is taken as a measure of the efficiency with which molecular clouds form, the radial decrease in the H₂ / HI ratio indicates that molecular cloud formation proceeds most efficiently or that the clouds last longer toward the center of the galaxy. Inefficient molecular cloud formation in the outer parts of galaxies could result in part from decreasing gas volume densities as the HI scale height increases (Mihalas & Binney 1981).

Models for the star formation efficiency within galactic disks have been proposed by Gusten & Mezger (1982) and Dopita (1985). The density wave model of Gusten & Mezger predicts that the SFE should depend on the difference between the angular velocity and the pattern speed, [ (R) - _p], which decreases as a function of radius. Dopita's model also depends on the pressure of the ISM and predicts a SFE that is proportional to the stellar surface density, also decreasing with radius. The predictions of these models have been compared with the observationally derived SFE for M51 (Lord & Young 1990), but neither model accounts for the apparent constancy of the SFE with radius.