Reduction of the type Ia supernova host galaxy step in the outer regions of galaxies

Toy, M and Wiseman, P and Sullivan, M and Scolnic, D and Vincenzi, M and Brout, D and Davis, T M and Frohmaier, C and Galbany, L and Lidman, C and Lee, J and Kelsey, L and Kessler, R and Möller, A and Popovic, B and Sánchez, B O and Shah, P and Smith, M and Allam, S and Aguena, M and Alves, O and Bacon, D and Brooks, D and Burke, D L and Rosell, A Carnero and Carretero, J and da Costa, L N and Pereira, M E S and Desai, S and Diehl, H T and Doel, P and Drlica-Wagner, A and Everett, S and Ferrero, I and Flaugher, B and Frieman, J and García-Bellido, J and Gatti, M and Gaztanaga, E and Giannini, G and Gruendl, R A and Gutierrez, G and Hinton, S R and Hollowood, D L and Honscheid, K and James, D J and Lahav, O and Lee, S and Marshall, J L and Mena-Fernández, J and Miquel, R and Palmese, A and Pieres, A and Malagón, A A Plazas and Romer, A K and Samuroff, S and Sanchez, E and Cid, D Sanchez and Schubnell, M and Suchyta, E and Swanson, M E C and Tarle, G and Tucker, D L and Vikram, V and Walker, A R and Weaverdyck, N (2025) Reduction of the type Ia supernova host galaxy step in the outer regions of galaxies. Monthly Notices of the Royal Astronomical Society, 538 (1). pp. 181-197. ISSN 0035-8711

Abstract

Using 1533 type Ia supernovae (SNe Ia) from the five-year sample of the Dark Energy Survey (DES), we investigate the relationship between the projected galactocentric separation of the SNe and their host galaxies and their light curves and standardization. We show, for the first time, that the difference in SN Ia post-standardization brightnesses between high and low-mass hosts reduces from 0.078 ± 0.011 mag in the full sample to 0.036 ± 0.018 mag for SNe Ia located in the outer regions of their host galaxies, while increasing to 0.100 ± 0.014 mag for SNe in the inner regions. The difference in the size of the mass step between inner and outer regions is 0.064 ± 0.023 mag. In these inner regions, the step can be reduced (but not removed) using a model where the RV of dust along the line-of-sight to the SN changes as a function of galaxy properties. We investigate the remaining difference using the distributions of the SN Ia stretch parameter to test the inferred age of SN progenitors. Comparing red (older) environments only, outer regions have a higher proportion of high-stretch SNe and a more homogeneous stretch distribution. However, this effect cannot explain the reduction in significance of any Hubble residual step in outer regions. We conclude that the standardized distances of SNe Ia located in the outer regions of galaxies are less affected by their global host galaxy properties than those in the inner regions.