THE DISTRIBUTION OF THE INTERSTELLAR-MEDIUM IN THE SCD GALAXY NGC-6946 .3. AZIMUTHAL VARIATIONS IN THE MOLECULAR AND ATOMIC COMPONENTS

We present here a comparison of the azimuthal variations in the H2 and H I surface densities with those in B- and I-band images (Elmegreen and Elmegreen, private communication) and Hα fluxes (Bonnarel, Boulesteix and Marcelin) to gain insight into how the presence of a spiral potential affects the gas surface densities and the massive star formation efficiencies in NGC 6946. In regions of well-defined spiral structure in the northeast quadrant of the galaxy, we find enhancements in the H2 surface densities, Ha surface brightnesses, and the μ(Hα)/σ(H2) ratio on the spiral arms. In the southwest part of the galaxy where the spiral structure is poorly defined by the optical images, enhancements in the H2 surface density are not as well correlated with enhancements in the Hα flux or the μ(Hα)/σ(H2) ratio. We interpret the Hα/H2 ratio as a measure of the massive star formation efficiency and infer that this efficiency is greater on the spiral arms than in the interarm regions of NGC 6946. Radial increases in the spiral arm amplitudes, as derived in the B and I bands, are evident. Correspondingly, large radial increases in the arm-interarm contrasts of the Hα surface brightness and massive star formation efficiency are also prominent, with smaller increases observed in the H2 surface density arm-interarm contrasts. We suggest that the massive star formation efficiency is a sensitive function of spiral arm strength.