  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.
