Sample of 141 SNe Ia

This is the strongest standardized SN Ia brightness systematic connected to the host-galaxy environment measured to date. The well-established step in standardized brightnesses between SNe Ia in hosts with lower or higher total stellar masses is smaller, at {DELTA}M=0.119+/-0.032mag (4.5{sigma}), for the same set of SNe Ia. When fit simultaneously, the environment-age offset remains very significant, with {DELTA}Y=0.129+/-0.032mag (4.0{sigma}), while the global stellar mass step is reduced to {DELTA}M=0.064+/-0.029mag (2.2{sigma}).

Thus, approximately 70% of the variance from the stellar mass step is due to an underlying dependence on environment-based progenitor age. Also, we verify that using the local star formation rate alone is not as powerful as LsSFR at sorting SNe Ia into brighter and fainter subsets. Standardization that only uses the SNe Ia in younger environments reduces the total dispersion from 0.142+/-0.008mag to 0.120+/-0.010mag.

We show that as environment-ages evolve with redshift, a strong bias, especially on the measurement of the derivative of the dark energy equation of state, can develop. Fortunately, data that measure and correct for this effect using our local specific star formation rate indicator, are likely to be available for many next-generation SN Ia cosmology experiments.