The Opacity of Spiral Galaxy Disks IX; Dust and Gas Surface Densities

TL;DR

This study investigates the relationship between dust, atomic, and molecular gas in spiral galaxy disks, revealing a high dust-to-gas ratio that varies with galactic radius and is not solely linked to molecular clouds.

Contribution

It introduces a novel method combining optical disk opacity and sub-mm data to estimate dust content and examines its correlation with gas surface densities in spiral galaxies.

Findings

Dust-to-gas ratio is approximately 0.04.

Opacity does not correlate strongly with atomic or molecular gas alone.

Dust-to-gas ratio declines with galactocentric radius.

Abstract

Our aim is to explore the relation between gas, atomic and molecular, and dust in spiral galaxies. Gas surface densities are from atomic hydrogen and CO line emission maps. To estimate the dust content, we use the disk opacity as inferred from the number of distant galaxies identified in twelve HST/WFPC2 fields of ten nearby spiral galaxies. The observed number of distant galaxies is calibrated for source confusion and crowding with artificial galaxy counts and here we verify our results with sub-mm surface brightnesses from archival Herschel-SPIRE data. We find that the opacity of the spiral disk does not correlate well with the surface density of atomic (Hi) or molecular hydrogen (H2) alone implying that dust is not only associated with the molecular clouds but also the diffuse atomic disk in these galaxies. Our result is a typical dust-to-gas ratio of 0.04, with some evidence that this ratio declines with galactocentric radius, consistent with recent Herschel results. We discuss the possible causes of this high dust-to-gas ratio; an over-estimate of the dust surface-density, an under-estimate of the molecular hydrogen density from CO maps or a combination of both. We note that while our value of the mean dust-to-gas ratio is high, it is consistent with the metallicity at the measured radii if one assumes the Pilyugin & Thuan calibration of gas metallicity.