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

7.2 Arm-Interarm Variations

Azimuthal variations in NGC 6946 and M51 have been analyzed with the goal of determining the ratio of young stars to gas and H₂ / HI in spiral arm and interarm regions (Lord 1987, Tacconi 1987, Tacconi & Young 1990, Lord & Young 1990). Figure 10 shows the distributions of H₂, HI, H, B-, and I-band light, H / H₂, H₂ / HI, and H / blue as a function of azimuth in NGC 6946 at a radius of 1.5'. At this radius, the H arms at 45" resolution stand out as a factor of ~ 4 enhancements relative to interarm locations; in CO at this resolution, the spiral arms are much less apparent. Thus, the yield of massive stars per unit mass of molecular gas in NGC 6946 is enhanced by more than a factor of 2 on the spiral arms relative to the interarm regions. Tacconi & Young find the arm-interarm contrast in the H₂ / H to increase with radius to a value that exceeds 10 beyond a radius of 2'.

Figure 10. Azimuthal distributions of H2 HI, H, B-band, H / H2, H2 / HI, and H / blue at a radius of 1.5' in the disk of the Scd galaxy NGC 6946 from Tacconi & Young (1990), all derived at 45" resolution. Azimuths are defined from north through east, with north at 0°. The solid horizontal lines in the lower portion of each panel indicate the locations of the I-hand spiral arms before smoothing the data to 45" resolution. The H / H2 ratio as a function of azimuth shows enhancements in the spiral arm regions indicating that the star formation efficiency is a function of the H2 density.

Like NGC 6946, M51 also exhibits a factor of ~ 2 enhancement in the star formation efficiency on the spiral arms (Lord 1987, Vogel et al 1988, Lord & Young 1990). In particular, Vogel et al (1988) inferred arm/interarm contrasts of 2-5 in CO and 10 in H on scales of ~ 500 pc. A similar arm/interarm contrast was measured for CO in M51 by Garcia-Burillo & Guelin (1990). These studies indicate that the yield of high mass stars per unit H₂ mass in spiral arms of luminous Sc galaxies is enhanced relative to the interarm regions by at least a factor of 2.

The elevated H / CO ratios associated with spiral arm gas concentrations indicate that there is a nonlinear dependence of the OB star formation rate on the local gas surface density. The variations on and off the spiral arms in NGC 6946 and M51 are consistent with the high mass SFR depending quadratically on the H₂ surface density. Based on a similar increase in the number of radio H II regions relative to the density of GMCs in the spiral arms of the Galaxy, Scoville et al (1986a) suggested that OB star formation may be linked to the collisions of GMCs. A bias for the formation of high mass stars during cloud-cloud collisions could arise, since the gas will be shock heated and the first stars to form as the gas cools would be high mass stars (cf. Silk 1987). It is not clear whether the IMF within the arms is biased toward high mass stars, or whether more stars of all masses form in these regions. We suspect that there may well be separate modes of star formation for low and high mass stars (cf. Larson 1987). Perhaps the low mass star formation rate depends linearly on the gas density and the high mass mode quadratically.

How is it, then that the azimuthally averaged H and CO radial distributions are so similar in NGC 6946 (and M51), while the H shows greater enhancement on the spiral arms? This inconsistency could be resolved if the spiral arms comprise a small fraction of the surface area of a galaxy and only a small fraction of the total molecular gas and H resides in the arms. Thus, spiral arms are best studied by investigating gas and star formation as a function of azimuth, whereas azimuthal averaging reveals the characteristics of the underlying disk.