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

Abstract

We relate the optical attenuation inferred by the Balmer decrement, $A_{V,\mathrm{BD}}$, and by the SED-fitting, $A_{V,\mathrm{SED}}$, to the dust distribution and gas surface density throughout the disc of galaxies, down to scales smaller than 0.5 kpc. We investigate five nearby star-forming spirals with available FUV to sub-mm observations, along with atomic and molecular gas surface density maps and optical integral-field spectroscopic data. We use the CIGALE SED-fitting code to map the dust mass surface density (${\Sigma }_{\mathrm{dust}}$) and $A_{V,\mathrm{SED}}$ of different stellar populations. For each pixel, we independently estimate the attenuation from the BD. We find that both ${\Sigma }_{\mathrm{dust}}$ and $A_{V,\mathrm{BD}}$ trace better the molecular and total gas mass surface density, rather 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 young stars, $A_{V,\mathrm{SED}}^{\mathrm{young}}$, is in good agreement with $A_{V,\mathrm{BD}}$ and it can then be used to trace the attenuation in star forming galaxies where integral-field observations are not available. We estimate the ratio of $A_{V,\mathrm{BD}}$ over the total stellar $A_{V,\mathrm{SED}}$ and find it slightly larger than what has been found in previous studies. Finally, we investigate which dust distribution reproduces better the estimated $A_{V,\mathrm{BD}}$ and $A_{V,\mathrm{SED}}$. 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 $A_{V,\mathrm{BD}}$ and the $A_{V,\mathrm{SED}}^{\mathrm{young}}$ are between the values expected for a foreground dust screen and a mixed configuration.

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