  As part of an on-going effort to identify, understand and correct for
astrophysics biases in the standardization of Type Ia supernovae (SNIa) for
cosmology, we have statistically classified a large sample of nearby SNeIa into
those located in predominantly younger or older environments. This
classification is based on the specific star formation rate measured within a
projected distance of 1kpc from each SN location (LsSFR). This is an important
refinement compared to using the local star formation rate directly as it
provides a normalization for relative numbers of available SN progenitors and
is more robust against extinction by dust. We find that the SNeIa in
predominantly younger environments are DY=0.163\pm0.029 mag (5.7 sigma) fainter
than those in predominantly older environments after conventional light-curve
standardization. This is the strongest standardized SN Ia brightness systematic
connected to host-galaxy environment measured to date. The well-established
step in standardized brightnesses between SNeIa in hosts with lower or higher
total stellar masses is smaller at DM=0.119\pm0.032 mag (4.5 sigma), for the
same set of SNeIa. When fit simultaneously, the environment age offset remains
very significant, with DY=0.129\pm0.032 mag (4.0 sigma), while the global
stellar mass step is reduced to DM=0.064\pm0.029 mag (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. Standardization
using only the SNeIa in younger environments reduces the total dispersion from
0.142\pm0.008 mag to 0.120\pm0.010 mag. We show that as environment ages evolve
with redshift a strong bias on measurement of the dark energy equation of state
parameters can develop. Fortunately, data to measure and correct for this
effect is likely to be available for many next-generation experiments.
[abstract shorten]
