Redshift Evolution of the Underlying Type Ia Supernova Stretch Distribution. (arXiv:2005.09441v1 [astro-ph.CO])
<a href="http://arxiv.org/find/astro-ph/1/au:+Nicolas_N/0/1/0/all/0/1">N. Nicolas</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rigault_M/0/1/0/all/0/1">M. Rigault</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Copin_Y/0/1/0/all/0/1">Y. Copin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Graziani_R/0/1/0/all/0/1">R. Graziani</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Aldering_G/0/1/0/all/0/1">G. Aldering</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Briday_M/0/1/0/all/0/1">M. Briday</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nordin_J/0/1/0/all/0/1">J. Nordin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kim_Y/0/1/0/all/0/1">Y.-L. Kim</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Perlmutter_S/0/1/0/all/0/1">S. Perlmutter</a>
The true nature of type Ia supernovae (SNe Ia) remains largely unknown, and
as survey statistics increase, the question of astrophysical systematic
uncertainties rises, notably that of the SN Ia population evolution. In this
paper, we study the dependence with redshift of the SN Ia SALT2.4 lightcurve
stretch, a purely intrinsic SN property, to probe its potential redshift drift.
The SN stretch has been shown to strongly correlate with the SN environment,
notably with stellar age tracers. We model the underlying stretch distribution
as a function of redshift, using the evolution of fraction of young and old SNe
Ia as predicted by Rigault et al. (2018), and assuming constant underlying
stretch distribution for each age population made of Gaussian mixtures. We test
our prediction against published samples chosen to have negligible magnitude
selection effects, so that any observed change is indeed of astrophysical and
not observational origin. We clearly demonstrate that the underlying SN Ia
stretch distribution is evolving as a function of redshift, and that the
young/old drifting model is a much better description of the data than any
time-constant model, including the sample-based asymmetric distributions
usually used to correct Malmquist bias. The favored underlying stretch model is
the bimodal one derived from Rigault et al. (2018): a high-stretch mode shared
by both young and old environments, and a low-stretch mode exclusive to old
environments. The precise impact of the redshift evolution of the SN Ia
population intrinsic properties on cosmology remains to be studied. Yet,the
astrophysical drift of the SN stretch distribution does affect current
Malmquist bias corrections and thereby distances derived from SN affected by
selection effects. We highlight that such a bias will increase with surveys
covering increasingly larger redshift ranges, which is particularly important
for LSST.
The true nature of type Ia supernovae (SNe Ia) remains largely unknown, and
as survey statistics increase, the question of astrophysical systematic
uncertainties rises, notably that of the SN Ia population evolution. In this
paper, we study the dependence with redshift of the SN Ia SALT2.4 lightcurve
stretch, a purely intrinsic SN property, to probe its potential redshift drift.
The SN stretch has been shown to strongly correlate with the SN environment,
notably with stellar age tracers. We model the underlying stretch distribution
as a function of redshift, using the evolution of fraction of young and old SNe
Ia as predicted by Rigault et al. (2018), and assuming constant underlying
stretch distribution for each age population made of Gaussian mixtures. We test
our prediction against published samples chosen to have negligible magnitude
selection effects, so that any observed change is indeed of astrophysical and
not observational origin. We clearly demonstrate that the underlying SN Ia
stretch distribution is evolving as a function of redshift, and that the
young/old drifting model is a much better description of the data than any
time-constant model, including the sample-based asymmetric distributions
usually used to correct Malmquist bias. The favored underlying stretch model is
the bimodal one derived from Rigault et al. (2018): a high-stretch mode shared
by both young and old environments, and a low-stretch mode exclusive to old
environments. The precise impact of the redshift evolution of the SN Ia
population intrinsic properties on cosmology remains to be studied. Yet,the
astrophysical drift of the SN stretch distribution does affect current
Malmquist bias corrections and thereby distances derived from SN affected by
selection effects. We highlight that such a bias will increase with surveys
covering increasingly larger redshift ranges, which is particularly important
for LSST.
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