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

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
² European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
³ Scuola Normale Superiore, Piazza dei Cavalieri 7, 50126 Pisa, Italy

^⋆ Corresponding author: evangelos.paspaliaris@inaf.it, edpaspaliaris@gmail.com

Received: 11 April 2025
Accepted: 15 August 2025

Abstract

Aims. 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.

Methods. 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.

Results. 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_V, SED^young, 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_V, SED^young are between the values expected for a foreground dust screen and the mixed configuration.