J. Rino-Silvestre, S. González-Gaitán, A. M. Mourão, J. C. Duarte, B. Pereira
Abstract
Context. Peculiar dust extinction laws have been reported for some type Ia supernovae (SNe) with the parameter RV 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 RV 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 λmax and obtain 2D maps for RV. When the fit to λmax at the location of SNe is poor, or impossible, an approximate Bayesian spatial inference method was employed to obtain an estimate of λmax using well-fitted neighboring locations. The estimated local RV for each SN was compared with published values from the supernova light curves. Results. We find RV 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 RV obtained from the analysis of some reddened SNIa light curves. The RV estimates obtained with BVRI photopolarimetry for the vicinity of SNe are statistically similar to the global RV we obtain for the host. Conclusions. The discrepancy between the local RV, inferred from photopolarimetry in the vicinity of SNe, and the RV 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.
Keywords
techniques polarimetric / dust / ism dust extinction / galaxies intergalactic medium
Astronomy & Astrophysics
Volume 703, Article Number A170, Number of pages 18
2025 November
