Extinction law variations and dust excitation in the spiral galaxy NGC 300

Abstract

We investigate the possible origin of the strong radial gradient in the ultraviolet-to-infrared ratio in the nearby late-type spiral galaxy NGC 300 and emphasize the importance of local variations in the geometry of the interstellar medium, concluding that they cannot be neglected with respect to metallicity effects. This analysis is based on a combination of maps obtained both from space using GALEX (1350 to 2750 Å) and Spitzer (3-160 μm) and from the ground (UBVRI, Hα, and Hβ). We select ionizing stellar clusters associated with H II regions of widely varying morphologies and derive their fundamental parameters from population synthesis fitting of their spectral energy distributions, carefully measured to eliminate local background emission accurately. From these fits, we conclude that the stellar extinction law is highly variable in the line of sight of young clusters of similar ages. In the particular model geometry that we consider most appropriate to the sampled regions, we checked that our findings are not significantly altered by the correct treatment of radiative transfer effects. The variations are systematic in nature, in the sense that extinction laws of the Milky Way or LMC type, with a prominent 2175 Å absorption feature and a modest far-ultraviolet slope, are associated with compact H II regions (the compactness being quantified in two different ways), while clusters surrounded by diffuse H II regions follow extinction laws of the 30 Doradus or SMC type, with a weaker 2175 Å feature and a steeper far-ultraviolet rise. The Calzetti starburst attenuation law, although most often degenerate with the 30 Doradus extinction law, overpredicts ionizing photon fluxes by large amounts. We also find that the extinction law variations are correlated with the column density of dust species emitting in the near- and mid-infrared. Finally, we briefly discuss the nebular to stellar extinction ratios and the excitation of aromatic band carriers, invalidating their claimed association with cold dust.