As part of an on-going effort to identify , understand and correct for astrophysics biases in the standardization of Type Ia supernovae ( SN Ia ) for cosmology , we have statistically classified a large sample of nearby SNe Ia 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 1 kpc from each SN location , ( LsSFR ) .
This is an important refinement compared to using the local star formation rate directly ( Rigault et al .
2013 ; 2015 ) , as it provides a normalization for relative numbers of available SN progenitors and is more robust against extinction by dust .
We find that the SNe Ia in predominantly younger environments are \Delta _ { Y } = 0.163 \pm 0.029 \mathrm { 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 SNe Ia in hosts with lower or higher total stellar masses is smaller , at \Delta _ { M } = 0.119 \pm 0.032 \mathrm { mag } ( 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 \pm 0.032 mag ( 4.0 \sigma ) , while the global stellar mass step is reduced to \Delta _ { M } = 0.064 \pm 0.029 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 .
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 using only the SNe Ia in younger environments reduces the total dispersion from 0.142 \pm 0.008 \mathrm { mag } to 0.120 \pm 0.010 \mathrm { mag } .
We show that as environment ages evolve with redshift , a strong bias , especially on measurement of the derivative of the dark energy equation of state , can develop .
Fortunately , data to measure and correct for this effect using our local specific star formation rate indicator is likely to be available for many next-generation SN Ia cosmology experiments .