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