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

¹ CENTRA, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal
² Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências, Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisbon, Portugal
³ European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago, Chile
⁴ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281, 9000 Gent, Belgium
⁵ Millennium Institute of Astrophysics MAS, Nuncio Monsenor Sotero Sanz 100, Off. 104, Providencia, Santiago, Chile
⁶ Institute of Space Sciences (ICE-CSIC), Universitat Autònoma de Barcelona, Campus UAB, Carrer de Can Magrans, E-08193 Barcelona, Spain
⁷ Institut d’Estudis Espacials de Catalunya (IEEC), Mediterranean Technology Park (PMT), Baix Llobregat Campus – UPC, E-08860 Castelldefels (Barcelona), Spain
⁸ Astronomical Observatory, Volgina 7, 11060 Belgrade, Serbia

^⋆ Corresponding author: joao.r.d.duarte@tecnico.ulisboa.pt

Received: 6 March 2025
Accepted: 27 June 2025

Abstract

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_Veff, ranging from 0.5 to 7, showing that the two are not analogous, even for local resolved observations. We find that R_Veff 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_Veff, and R_Veff observed for star-forming galaxies. An increase in dust density leads to higher R_Veff across all inclinations, which, assuming a correlation between stellar mass and dust density, explains the increase in R_Veff with mass observed in star-forming galaxies. We cannot explain the observed differences in R_Veff between star-forming (higher R_Veff) and quiescent (lower R_Veff) 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_Veff 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_Veff for low A_Veff regions appears to better approximate the extinction R_V.