# Think Global, Act Local: The Influence of Environment Age and Host Mass on Type Ia Supernova Light Curves

## TL;DR

This study investigates how the age of the host galaxy environment and local factors influence Type Ia supernova light curves, revealing significant correlations that impact cosmological measurements like the Hubble constant.

## Contribution

The paper introduces an improved Bayesian method to estimate host environment ages and demonstrates their significant effect on supernova luminosity residuals, refining understanding of supernova standardization.

## Key findings

- A 0.114 mag step in Hubble residuals at ~8 Gyr host age.
- A new principal component, PC₁, correlates strongly with residuals (4.7σ).
- At most a 1.3% impact on H₀ estimation.

## Abstract

The reliability of Type Ia supernovae (SNIa) may be limited by the imprint of their galactic origins. To investigate the connection between supernovae and their host characteristics, we developed an improved method to estimate the stellar population age of the host as well as the local environment around the site of the supernova. We use a Bayesian method to estimate the star formation history and mass weighted age of a supernova's environment by matching observed spectral energy distributions to a synthesized stellar population. Applying this age estimator to both the photometrically and spectroscopically classified Sloan Digital Sky Survey II supernovae (N=103) we find a $0.114 \pm 0.039~{\rm mag}$ `step' in the average Hubble residual at a stellar age of $\sim 8~\text{Gyr}$; it is nearly twice the size of the currently popular mass step. We then apply a principal component analysis on the SALT2 parameters, host stellar mass, and local environment age. We find that a new parameter, PC$_1$, consisting of a linear combination of stretch, host stellar mass, and local age, shows a very significant ($4.7\sigma$) correlation with Hubble residuals. There is a much broader range of PC$_1$ values found in the Hubble flow sample when compared with the Cepheid calibration galaxies. These samples have mildly statistically different average PC$_1$ values, at $\sim 2.5\sigma$, resulting in at most a 1.3% reduction in the evaluation of H$_0$. Despite accounting for the highly significant trend in SNIa Hubble residuals, there remains a 9% discrepancy between the most recent precision estimates of H$_0$ using SNIa and the CMB.