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

TL;DR

This work revisits the SN Ia mass-step by leveraging broad multi-wavelength photometry and nonparametric SFHs from Prospector to obtain more accurate host-galaxy masses. Despite higher, more accurate mass estimates, the SN Ia luminosity–host-mass trend remains largely intact, and a metallicity-dependent environmental effect emerges, with low-metallicity environments (Z $< -1\,Z_\odot$) potentially driving much of the correlation. The analysis argues that the chemical composition of SN environments plays a central role in standardized brightness, linking progenitor environments to luminosity and suggesting refinements to cosmological inferences such as $H_0$. By combining improved SED fitting with careful photometric zeropoint calibrations across optical to mid-IR data, the study provides a more robust framework for interpreting SN Ia standardization and its astrophysical drivers.

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 Type Ia supernova (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 use 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 re-examine the mass-step relation with a sample of SN Ia host galaxies complete in broadband optical (3000 Angstrom to 1 micron) and near-infrared (up to 1.8 micron) data, and in some cases including mid-infrared observations (up to 4.5 micron), using Prospector to derive nonparametric SFHs. We find that while the masses for these galaxies have indeed been underestimated, the overall trend in SN Ia luminosity versus host mass remains largely unchanged. We also uncover an environmental metallicity-dependent trend, in which low-metallicity galaxies (Z < -1 Zsol) may drive much of the observed SN Ia luminosity-mass correlation, suggesting that the chemical composition of the SN environment plays a central role in shaping the standardized brightness of SNe Ia.