The UV-to-FIR environments of nearby Type Ia supernovae with the DustPedia Galaxy Catalogue – I. Local versus global host properties

Abstract The ongoing use of Type Ia Supernovae (SNe Ia) as standardisable candles relies on an additional correction for the relationship between their standardised brightnesses and host galaxy properties. Currently, this correction is built on host stellar mass, typically derived from optical-only SED fitting, which recent analyses suggest may not fully capture the underlying environmental properties. This relationship is stronger when considering the regions closest to the SNe, rather than the galaxies as a whole. I use a sample of SNe Ia in DustPedia galaxies and CIGALE SED fitting to compare the local environments of 90 nearby SNe Ia against the global properties of their hosts. The UV-to-FIR coverage of DustPedia enables improved dust constraints, and helps break the age-dust degeneracy that limits optical-only studies; to my knowledge this coverage has not been previously applied to the sub-galactic regions around SNe Ia. Stellar mass and dust mass are lower locally simply because the aperture encloses less of the galaxy; however, for colour, sSFR, and dust attenuation, local values differ from the host average by more than measurement uncertainty alone allows, with attenuation showing the strongest discrepancy (p = 6.3 × 10−5). Only mass-weighted age is consistent locally and globally. Galaxy-integrated properties therefore do not capture the true conditions at the SN location. SN siblings (sharing a host galaxy) only emphasise this effect. Siblings sampling the same galaxy structure, such as two SNe in spiral arms, have consistent environments, whilst others diverge. Standardising on global properties may therefore mask the differences that drive the brightness-environment relationship.