A galaxy-driven model of type Ia supernova luminosity variations

TL;DR

This study explores how galaxy properties and progenitor ages influence Type Ia supernova luminosity variations, finding dust extinction effects and intrinsic luminosity steps are key factors, with implications for cosmology.

Contribution

It introduces a galaxy-based forward model linking progenitor age, dust extinction, and luminosity variations, improving understanding of supernova standardization.

Findings

Dust extinction ratio step explains host galaxy mass effects.

Intrinsic luminosity difference alone cannot account for observed trends.

Modeling dust effects as a function of galaxy age improves data fit.

Abstract

Type Ia supernovae (SNe Ia) are used as standardisable candles to measure cosmological distances, but differences remain in their corrected luminosities which display a magnitude step as a function of host galaxy properties such as stellar mass and rest-frame $U-R$ colour. Identifying the cause of these steps is key to cosmological analyses and provides insight into SN physics. Here we investigate the effects of SN progenitor ages on their light curve properties using a galaxy-based forward model that we compare to the Dark Energy Survey 5-year SN Ia sample. We trace SN Ia progenitors through time and draw their light-curve width parameters from a bimodal distribution according to their age. We find that an intrinsic luminosity difference between SNe of different ages cannot explain the observed trend between step size and SN colour. The data split by stellar mass are better reproduced by following recent work implementing a step in total-to-selective dust extinction ratio $(R_V)$ between low- and high-mass hosts, although an additional intrinsic luminosity step is still required to explain the data split by host galaxy $U-R$. Modelling the $R_V$ step as a function of galaxy age provides a better match overall. Additional age vs. luminosity steps marginally improve the match to the data, although most of the step is absorbed by the width vs. luminosity coefficient $\alpha$. Furthermore, we find no evidence that $\alpha$ varies with SN age.