Constraining R$_V$ Variation Using Highly Reddened Type Ia Supernovae from the Pantheon+ Sample

TL;DR

This study uses highly reddened Type Ia supernovae from the Pantheon+ sample to investigate dust extinction properties, finding no evidence for $R_V$ variation with colour and supporting a distribution of $R_V$ values.

Contribution

It provides the first comprehensive analysis of highly reddened SNe Ia to constrain $R_V$ variation, challenging previous claims of a decreasing $R_V$ with colour.

Findings

No significant change in colour-luminosity coefficient with reddening.

Data favors a flat $R_V$ dependence on colour.

Supports $R_V$ as a distribution rather than a single value.

Abstract

Type Ia supernovae (SNe Ia) are powerful tools for measuring the expansion history of the universe, but the impact of dust around SNe Ia remains unknown and is a critical systematic uncertainty. One way to improve our empirical description of dust is to analyse highly reddened SNe Ia ($E(B-V)>0.4$, roughly equivalent to the fitted SALT2 light-curve parameter $c>0.3$). With the recently released Pantheon+ sample, there are 57 SNe Ia that were removed because of their high colour alone (with colours up to $c=1.61$), which can provide enormous leverage on understanding line-of-sight $R_V$. Previous studies have claimed that $R_V$ decreases with redder colour, though it is unclear if this is due to limited statistics, selection effects, or an alternative explanation. To test this claim, we fit two separate colour-luminosity relationships, one for the main cosmological sample ($c<0.3$) and one for highly reddened ($c>0.3$) SNe Ia. We find the change in the colour-luminosity coefficient to be consistent with zero. Additionally, we compare the data to simulations with different colour models, and find that the data prefers a model with a flat dependence of $R_V$ on colour over a declining dependence. Finally, our results strongly support that line-of-sight $R_V$ to SNe Ia is not a single value, but forms a distribution.