Accuracy of environmental tracers and consequence for determining the Type Ia Supernovae magnitude step

TL;DR

This study evaluates how environmental indicators like local Hα-based specific star formation rate and galaxy mass influence the standardization of Type Ia Supernovae luminosity, revealing two distinct populations affecting cosmological measurements.

Contribution

It compares various environmental tracers for their effectiveness in distinguishing supernova populations, highlighting the superiority of lsSFR and galaxy mass in standardizing SNe Ia luminosity.

Findings

lsSFR and galaxy mass are better tracers than morphology.

Two supernova populations differ by at least 0.121 mag.

Using lsSFR explains observed step function discrepancies.

Abstract

Type Ia Supernovae (SNe Ia) are standardizable candles that allow us to measure the recent expansion rate of the Universe. Due to uncertainties in progenitor physics, potential astrophysical dependencies may bias cosmological measurements if not properly accounted for. The dependency of the intrinsic luminosity of SNe Ia with their host-galaxy environment is often used to standardize SNe Ia luminosity and is commonly parameterized as a step function. This functional form implicitly assumes two-populations of SNe Ia. In the literature, multiple environmental indicators have been considered, finding different, sometimes incompatible, step function amplitudes. We compare these indicators in the context of a two-populations model, based on their ability to distinguish the two populations. We show that local H$\alpha$-based specific star formation rate (lsSFR) and global stellar mass are better tracers than, for instance, host galaxy morphology. We show that tracer accuracy can explain the discrepancy between the observed SNe Ia step amplitudes found in the literature. Using lsSFR or global mass to distinguish the two populations can explain all other observations, though lsSFR is favoured. As lsSFR is strongly connected to age, our results favour a prompt and delayed population model. In any case, there exists two populations that differ in standardized magnitude by at least $0.121\pm0.010\,\mathrm{mag}$.