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Type Ia supernovae (SNe Ia) are standardizable candles, but for over adecade, there has been a debate on how to properly account for theircorrelations with host galaxy properties. Using the Bayesian hierarchical modelUNITY, we simultaneously fit for the SN Ia light curve and host galaxystandardization parameters on a set of 103 Sloan Digital Sky Survey II SNe Ia.We investigate the influences of host stellar mass, along with both localized($r<3$ kpc) and host-integrated average stellar ages, derived from stellarpopulation synthesis modeling. We find that the standardization for thelight-curve shape ($\alpha$) is correlated with host galaxy standardizationterms ($\gamma_i$) requiring simultaneous fitting. In addition, we find thatthese correlations themselves are dependent on host galaxy stellar mass thatincludes a shift in the color term ($\beta$) of $0.8 \mathrm{mag}$, onlysignificant at $1.2\sigma$ due to the small sample. We find a linear host massstandardization term at the $3.7\sigma$ level, that by itself does notsignificantly improve the precision of an individual SN Ia distance. However, astandardization that uses both stellar mass and average local stellar age isfound to be significant at $>3\sigma$ in the two-dimensional posterior space.In addition, the unexplained scatter of SNe Ia absolute magnitude poststandardization, is reduced from $0.122^{+0.019}_{-0.018}$ to $0.109\pm0.017$mag, or $\sim10\%$. We do not see similar improvements when using global ages.This combination is consistent with either metallicity or line-of-sight dustaffecting the observed luminosity of SNe Ia.

Host Galaxy Mass Combined with Local Stellar Age Improve Type Ia Supernovae Distances