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

TL;DR

This study revisits the relationship between Type Ia supernova luminosities and host galaxy properties, revealing that environmental metallicity significantly influences supernova brightness, beyond the previously observed mass step.

Contribution

It introduces nonparametric star formation histories and extended wavelength data to better estimate host galaxy masses, highlighting metallicity's role in supernova luminosity variations.

Findings

Mass estimates are more accurate with extended wavelength data.

The mass step persists despite improved mass estimates.

Low-metallicity environments may primarily drive the luminosity-mass correlation.

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.