  We present a new dust extinction technique with which we are able to retrieve
parsec-scale gas surface density maps for entire nearby galaxies. The method
measures the dust attenuation in optical bands on a pixel-by-pixel basis
against a smoothed, reconstructed stellar distribution. The contribution of
foreground light along the line-of-sight is calibrated using dust emission
observations, assuming that the dust sits in a layer close to the mid-plane of
the face-on galaxy. Here, we apply this technique to M51 (NGC 5194) as a
proof-of-concept, obtaining a resolution of 0.14" (5 pc). Our dust (and gas)
surface density map is consistent with independent dust- and CO-based studies
at lower resolution. We find that discrepancies between our estimates of
surface density and other studies stem primarily from the choice of dust model
(i.e. different dust absorption coefficients). When assuming the same dust
opacity law, our technique produces surface densities that are consistent with
independent studies. This dust extinction technique provides us with gas
surface density maps at an unprecedented resolution for full disc coverage
studies of nearby galaxies. The resulting well-resolved spatial information
opens the possibility for more in-depth examination of the influence of
large-scale dynamics (and also stellar feedback mechanisms) on the interstellar
medium at parsec-scales, and consequently star formation in nearby galaxies.
