Far-ultraviolet Dust Extinction and Molecular Hydrogen in the Diffuse Milky Way Interstellar Medium

TL;DR

This study investigates the relationship between far-ultraviolet dust extinction features and molecular hydrogen in the diffuse interstellar medium of the Milky Way, revealing a strong link between H$_2$ and the UV extinction rise.

Contribution

It establishes a direct correlation between molecular hydrogen content and the far-UV extinction rise, highlighting the molecular gas as the carrier of this dust feature.

Findings

Strong correlation between H$_2$ column density and far-UV extinction rise.

Far-UV rise does not scale with atomic hydrogen, but with molecular hydrogen.

Colder, denser sightlines show stronger far-UV rise features.

Abstract

We aim to compare variations in the full-UV dust extinction curve (912-3000 Angstrom), with the HI/H$_2$/total H content along diffuse Milky Way sightlines, to investigate possible connections between ISM conditions and dust properties. We combine an existing sample of 75 UV extinction curves based on IUE and FUSE data, with atomic and molecular column densities measured through UV absorption. The H$_2$ column density data are based on existing Lyman-Werner absorption band models from earlier work on the extinction curves. Literature values for the HI column density were compiled, and improved for 23 stars by fitting a Ly$\alpha$ profile to archived spectra. We discover a strong correlation between the H$_2$ column and the far-UV extinction, and the underlying cause is a linear relationship between H$_2$ and the strength of the far-UV rise feature. This extinction does not scale with HI, and the total H column scales best with $A(V)$ instead. The carrier of the far-UV rise therefore coincides with molecular gas, and further connections are shown by comparing the UV extinction features to the molecular fraction. Variations in the gas-to-extinction ratio $N(\rm{H})/A(V)$ correlate with the UV-to-optical extinction ratio, and we speculate this could be due to coagulation or shattering effects. Based on the H$_2$ temperature, the strongest far-UV rise strengths are found to appear in colder and denser sightlines.