No More Heterodyne Blues: Gas Masses From Millimeter and Sub-Millimeter Continuum Photometry

I argue that reliable extragalactic gas masses can be derived from millimeter and sub-millimeter photometry. On the basis of currently-available information, such calculated masses are more accurate, for many types of galaxies, than those determined from CO line observations. This accuracy is due to several factors: (l) in contrast to the J = 1 → 0 CO line in many cases, the dust emission is optically thin; (2) at very long far-infrared wavelengths, the dust emission is only linearly proportional to less-certain input parameters such as temperature; (3) a great deal has been accomplished in estimating values appropriate to the Milky Way for the most uncertain factor in converting long-wavelength dust emission into gas mass, the ratio N(H2)/τλ and (4) variations in this ratio for galaxies other than the Milky Way is very likely to be easier to estimate than is the case for analysis of molecular line data. Equally important, in my view, the conversion of long-wavelength dust emission into gas mass is extremely straightforward, with the systematic uncertainties of the conversion being clear and, thus, subject more to debate than to conjecture.