The distribution of the interstellar medium in the Scd galaxy NGC 6946 and in a sample of dwarf irregular galaxies.

Abstract

We have compared the atomic and molecular hydrogen distributions in the spiral galaxy NGC 6946 and in 15 dwarf irregular galaxies. We have made observations of the CO J = 1 $\to$ 0 transition to obtain a fully sampled map to a radium of 10 kpc in NGC 6946, using the 14-meter telescope of the Five College Radio Astronomy Observatory, and have also completely mapped the 21-cm line of neutral hydrogen with the Very Large Array. We have also made CO observations at FCRAO in 15 dwarf irregular galaxies spanning a wide range of infrared luminosity and color. In NGC 6946 the CO and HI maps were used to investigate the radial and azimuthal variations in the atomic and molecular gas. The H\sb2 mass is found to dominate the ISM in the inner disk of this galaxy. The H\sb2, H$\alpha$, radio continuum, and blue light radial profiles are found to be centrally peaked with exponential falloffs, while the HI distribution is much flatter, with a central depression. We infer a constant efficiency of star formation with radius in NGC 6946 and suggest that HI plays only a passive role in the star formation process of this galaxy. Spiral arm enhancements in the H\sb2 surface densities, H$\alpha$ surface brightnesses, and $\sigma$(H\sb2)/$\mu$(H$\alpha$) ratios are observed. The presence of these arm enhancements, as well as enhancements in the gas surface densities in interarm regions is consistent with a picture in which cloud-cloud collisions play an important role in the star formation process in NGC 6946. An observed strong increase in the $\mu$(H$\alpha$)/$\sigma$(H\sb2) ratio with increasing arm amplitude indicates that the massive star formation efficiency is a sensitive function of spiral arm strength. For the dwarf galaxies, high observed L\sb{\rm IR}/L\sb{\rm CO} ratios in the blue systems are indicative of a present burst of star formation, while the red systems exhibit low L\sb{\rm IR}/L\sb{\rm CO} ratios. The M(H\sb2)/M(HI) ratios are smaller in the blue dwarfs than in either the red systems or in NGC 6946. We suggest that a low metallicity, low volume density, and an increased UV radiation field are responsible for the low H\sb2/HI ratios in active dwarf galaxies