R_V from multiwaveband galaxy polarimetry in the vicinity of supernovae

¹ CENTRA, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 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

^★ Corresponding authors: joao.silvestre@tecnico.ulisboa.pt; amourao@tecnico.ulisboa.pt; gongsale@gmail.com

Received: 6 February 2025
Accepted: 10 September 2025

Abstract

Context. Peculiar dust extinction laws have been reported for some type Ia supernovae (SNe) with the parameter R_V much lower than the average value of 3.1 for the Milky Way. These findings challenge our understanding of dust properties in galaxies, carrying unknown implications for supernovae cosmology.

Aims. Using optical photopolarimetry of supernova host galaxies, a few years after the explosion, we estimate R_V in the vicinity of each supernova and compare it with the extinction law calculated directly from observations of SNe.

Methods. Multiband photopolarimetric data of nine galaxies, hosts of eleven SNe, acquired with VLT-FORS2 in IPOL mode, were used to map the polarization angle and the polarization degree in each galaxy. Data were processed with a custom-built reduction pipeline that corrects for instrumental, background, and Milky Way interstellar polarization effects. The validity of Serkowski relations was tested at different locations in the galaxy to extract the wavelength of the maximum polarization λ_max and obtain 2D maps for R_V. When the fit to λ_max at the location of SNe is poor, or impossible, an approximate Bayesian spatial inference method was employed to obtain an estimate of λ_max using well-fitted neighboring locations. The estimated local R_V for each SN was compared with published values from the supernova light curves.

Results. We find R_V values from optical photopolarimetry at locations of SNe consistent with the average Milky Way value and a median difference of 2.5σ with the low peculiar R_V obtained from the analysis of some reddened SNIa light curves. The R_V estimates obtained with BVRI photopolarimetry for the vicinity of SNe are statistically similar to the global R_V we obtain for the host.

Conclusions. The discrepancy between the local R_V, inferred from photopolarimetry in the vicinity of SNe, and the R_V obtained from the light curves of SNe suggests that the extinction laws obtained directly from the SNe may be driven by more local effects, perhaps due to supernova light interactions with very nearby material.