Revisiting the Mass Step: Environmental Dependence of Type Ia Supernovae in Low-metallicity Host Galaxies

Abstract Despite the tremendous impact of Type Ia supernovae (SNe Ia) on the field of cosmology, their underlying physics are still poorly understood. Studies have found an intriguing correlation between standardized SN Ia luminosities and host galaxy masses, commonly referred to as the “mass-step.” SNe Ia in massive galaxies appear systematically brighter than in lower-mass, star-forming hosts after standardization. However, previous analyses utilize host galaxy mass estimates derived largely from optical data alone and assume parametric forms for host star formation histories (SFHs), both of which are known to misestimate galaxy stellar masses. In this work we reexamine the mass-step relation with a sample of SN Ia host galaxies complete in broadband optical (∼3000Å–1 μ m) and near-IR (∼1–1.8 μ m) and in some cases, with data up to the mid-IR (to ∼4.5 μ m), using prospector to derive nonparametric SFHs. We find that while the masses for these sample galaxies have indeed been underestimated, the overall trend in SN Ia luminosity versus host mass remains largely unchanged. However, we also uncover an environmental metallicity-dependent trend, in which low-metallicity galaxies (Z < −1 Z ⊙ ) may drive much of the observed SN Ia luminosity–mass correlation, suggesting that chemical composition of the SN environment may play a central role in shaping the standardized brightness of SNe Ia.