UV Extinction as a More Fundamental Measure of Dust than E(B-V) or A(V)

TL;DR

This study demonstrates that far-UV extinction at around 2900 Å is a more fundamental and consistent measure of dust column density in the Milky Way than traditional metrics like E(B-V) or A(V), by analyzing full extinction curves.

Contribution

The paper introduces the use of full extinction curves to identify the wavelength where dust measurements are most linear and minimizes scatter, establishing far-UV extinction as a superior dust tracer.

Findings

Extinction at 2900 Å minimizes scatter with N(H) and is most linear.

Near-UV extinction provides a more reliable measure of dust column density.

The discrepancy in gas-to-dust ratios between MW and SMC is reduced when using far-UV extinction.

Abstract

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 was truly universal, the strength of the relationship and the linearity would naturally be maintained for extinction at any wavelength, and also for N(H) vs. E(B-V). However, extinction curves vary within the Milky Way, and there is no reason why, except by chance, 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 and find that the scatter between N(H) and extinction is minimized -- and the relation becomes linear -- for extinction at 2900 +/- 160 A. 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, non-linear 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 of the gas in the gas-to-dust ratio, even though it may not be a convenient one.