Relating spatially resolved optical attenuation, dust, and gas in nearby galaxies

The purpose of the present study is to relate the optical attenuation inferred by the Balmer decrement, A_ V BD and by the spectral energy distribution (SED) fitting, A_ V SED to the dust distribution and gas surface density throughout the disc of galaxies, down to scales smaller than 0. 5 kpc. We investigated five nearby Herschel -detected star-forming spiral galaxies with available far-ultraviolet to sub-millimetre observations, along with atomic and molecular gas surface density maps and optical integral-field spectroscopic data. We used the CIGALE SED-fitting code to map the dust mass surface density (Σ _ dust) and A_ V SED of different stellar populations. For each pixel, we independently estimated the attenuation from the Balmer decrement. We find that both Σ _ dust and A_ V BD are better at tracing the molecular and total gas mass surface density than the atomic gas. Since regions sampled in this study have high molecular fractions, atomic gas surface densities, indicative of molecular gas shielding layers, decrease as the mean dust-to-gas ratio increases from galaxy to galaxy. The fitted attenuation towards the young stellar population, A^ young V SED is in good agreement with A_ V BD. It can then be used to trace the attenuation in star-forming galaxies for which integral-field observations are not available. We estimate the ratio of A_ V BD over the total stellar A_ V SED and find it slightly larger than what has been found in previous studies. Finally, we investigate which dust distribution better reproduces the estimated A_ V BD and A_ V SED. We find that the attenuation towards old stars is consistent with the expectations for a standard galactic disc, where the stellar and dust distributions are mixed, while A_ V BD and A^ young V SED are between the values expected for a foreground dust screen and the mixed configuration.