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 ( H i ) or molecular hydrogen ( { H _ { 2 } } ) 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 .