Variations between Dust and Gas in the Diffuse Interstellar Medium. 3. Changes in Dust Properties

TL;DR

This study investigates dust property variations in diffuse interstellar clouds, finding that observed low gas-to-dust ratios are mainly due to molecular hydrogen ('dark gas') rather than dust changes.

Contribution

It provides new insights into dust property variations and their impact on gas-to-dust ratio measurements in diffuse clouds, emphasizing the role of dark molecular gas.

Findings

Infrared emission decreases with higher dust optical depth.

Dust temperature drops in denser regions, mainly due to extinction.

Low gas-to-dust ratios are explained by dark molecular hydrogen.

Abstract

We study infrared emission of 17 isolated, diffuse clouds with masses of order solar masses, to test the hypothesis that grain property variations cause the apparently low gas-to-dust ratios that have been measured in those clouds. Maps of the clouds were constructed from WISE data and directly compared to the maps of dust optical depth from Planck. The mid-infrared emission per unit dust optical depth has a significant trend toward lower values at higher optical depths. The trend can be quantitatively explained by extinction of starlight within the clouds. The relative amounts of PAH and very small grains traced by WISE, compared to large grains tracked by Planck, are consistent with being constant. The temperature of the large grains significantly decreases for clouds with larger dust optical depth; this trend is partially due to dust property variations but is primarily due to extinction of starlight. We updated the prediction for molecular hydrogen column density, taking into account variations in dust properties, and find it can explain the observed dust optical depth per unit gas column density. Thus the low gas-to-dust ratios in the clouds are most likely due to `dark gas' that is molecular hydrogen.