Estimating gas masses from HI and CO data

The technique exploits the sensitivity of certain optical lines to changes in depletion of metals on to dust grains and uses photoionization models to constrain these physical ratios along with the metallicity and dust column density. We compare our gas column density estimates with HI and CO gas mass estimates in nearby galaxies to show that we recover their total gas mass surface density to within a factor of 2 up to a total surface gas mass density of ~75M{sun}/pc^2^.

Our technique is independent of the conversion factor of CO to H_2_ and we show that a metallicity-dependent X_CO_ is required to achieve good agreement between our measurements and that provided by CO and HI. However, we also show that our method cannot be reliably aperture corrected to total integrated gas mass. We calculate dust-to-gas ratios for all star-forming galaxies in the Sloan Digital Sky Survey Data Release 7 and show that the resulting dependence on metallicity agrees well with the trend inferred from modelling of the dust emission of nearby galaxies using far-IR data.