2019ApJ...874...32R

The reliability of Type Ia Supernovae (SNe Ia) 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 ± 0.039 mag "step" in the average Hubble residual at a stellar age of ∼8 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, PC₁, consisting of a linear combination of stretch, host stellar mass, and local age, shows a very significant (4.7σ) correlation with Hubble residuals. There is a much broader range of PC₁ values found in the Hubble flow sample when compared with the Cepheid calibration galaxies. These samples have mildly statistically different average PC₁ values, at ∼2.5σ, resulting in at most a 1.3% reduction in the evaluation of H₀. Despite accounting for the highly significant trend in SN Ia Hubble residuals, there remains a 9% discrepancy between the most recent precision estimates of H₀ using SN Ia and the CMB.