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# 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. ^

Astronomy

#### Recommended Citation

Tacconi-Garman, Linda Jean, "The distribution of the interstellar medium in the Scd galaxy NGC 6946 and in a sample of dwarf irregular galaxies" (1988). Doctoral Dissertations Available from Proquest. AAI8822863.
https://scholarworks.umass.edu/dissertations/AAI8822863

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