Atomic and Molecular Gas in Disk Galaxies
Current knowledge of the radial distributions of atomic and molecular gas in disk galaxies is briefly reviewed. Almost all of our knowledge is based on observations of the Hi and CO lines at 21 cm and 2.6 mm wavelength, and some of the caveats associated with these methods are discussed. In nearby spiral galaxies the molecular gas fraction is observed to decrease with radius, which can be understood in terms of a decline in hydrostatic disk pressure. Within the LMC, the CO-Hi correlation shows considerable scatter on scales of ~50 pc, although a binning analysis shows a strong non-linear dependence of CO on Hi intensity. The implications for molecular cloud formation and star formation recipes are briefly discussed.
KeywordsStar Formation Molecular Cloud Column Density Spiral Galaxy Disk Galaxy
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