2021ApJ...911...40B

The gas-to-dust ratio of reddened stars in the Milky Way (MW), the Magellanic Clouds, and in general, is usually expressed as a linear relation between the hydrogen column density, N(H), and the reddening, E(B - V), or extinction in the V-band (A_V). If the extinction curve were truly universal, the strength of the relationship and the linearity would naturally be maintained for extinction at any wavelength, as well as for N(H) versus E(B - V). However, extinction curves vary within the MW, and there is no reason, except by chance, why either E(B - V) or A_V would be the most physical measure of dust column density. In this paper, we utilize for the first time full extinction curves to 41 MW sightlines, finding that the scatter between N(H) and extinction is minimized-and the relation becomes linear-for extinction at 2900 ± 160 Å. Scatter and nonlinearity increase at longer wavelengths, and are especially large for near-IR extinction. We conclude that near-UV extinction is a superior measure of the dust column density for MW dust. We provide new, nonlinear gas-to-dust relations for various dust tracers. We also find that the very large discrepancy between MW and SMC gas-to-dust ratios of 0.9 dex in N(H)/E(B - V) is reduced to 0.7 dex for far-UV extinction, which matches the difference in cosmic abundances of carbon between the two galaxies, and therefore confirms that N(C) is the preferred measure for gas in the gas-to-dust ratio, even though it may not be a convenient one.