Assessing differences between local dust attenuation and point source extinction within the same galactic environments

Context. Dust attenuation in galaxies has often been used as a proxy for the extinction of point sources, such as supernovae, even though this approach ignores fundamental differences between the two cases. Aims. We present an analysis of the impact of geometric effects and scattering within dusty media on recovered galaxy dust properties. Methods. We employed SKIRT, a radiative transfer code, to simulate observations of point sources embedded in dust clouds, as well as spiral and elliptical galaxies. We examined various galaxy morphologies, inclinations, and instrument apertures. Results. We find that in galaxies the scattering of light into the line of sight and the presence of sources at different depths within the galaxy make attenuation fundamentally different from extinction. For a medium with an intrinsic extinction curve slope of R V = 3.068, we recover effective attenuation curve slopes, R V eff , ranging from 0.5 to 7, showing that the two are not analogous, even for local resolved observations. We find that R V eff greatly depends on dust density, galaxy morphology, and inclination, the latter being the most significant. A single simulated galaxy, viewed from different angles, can qualitatively reproduce the well-known relation between attenuation strength, A V eff , and R V eff observed for star-forming galaxies. An increase in dust density leads to higher R V eff across all inclinations, which, assuming a correlation between stellar mass and dust density, explains the increase in R V eff with mass observed in star-forming galaxies. We cannot explain the observed differences in R V eff between star-forming (higher R V eff ) and quiescent (lower R V eff ) high-mass galaxies, suggesting intrinsically lower R V values for ellipticals. Conclusions. We conclude that highly attenuated regions of simulated face-on galaxies yield R V eff within a 10% error of the intrinsic extinction R V of the medium, allowing different dust types to be distinguished. For edge-on spirals, the median R V eff for low A V eff regions appears to better approximate the extinction R V .