Early-type Host Galaxies of Type Ia Supernovae. II. Evidence for Luminosity Evolution in Supernova Cosmology. (arXiv:1912.04903v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Kang_Y/0/1/0/all/0/1">Yijung Kang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lee_Y/0/1/0/all/0/1">Young-Wook Lee</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kim_Y/0/1/0/all/0/1">Young-Lo Kim</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Chung_C/0/1/0/all/0/1">Chul Chung</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ree_C/0/1/0/all/0/1">Chang Hee Ree</a>
The most direct and strongest evidence for the presence of dark energy is
provided by the measurement of galaxy distances using type Ia supernovae (SNe
Ia). This result is based on the assumption that the corrected brightness of SN
Ia through the empirical standardization would not evolve with look-back time.
Recent studies have shown, however, that the standardized brightness of SN Ia
is correlated with host morphology, host mass, and local star formation rate,
suggesting a possible correlation with stellar population property. In order to
understand the origin of these correlations, we have continued our
spectroscopic observations to cover most of the reported nearby early-type host
galaxies. From high-quality (signal-to-noise ratio ~175) spectra, we obtained
the most direct and reliable estimates of population age and metallicity for
these host galaxies. We find a significant correlation between SN luminosity
(after the standardization) and stellar population age at a 99.5% confidence
level. As such, this is the most direct and stringent test ever made for the
luminosity evolution of SN Ia. Based on this result, we further show that the
previously reported correlations with host morphology, host mass, and local
star formation rate are most likely originated from the difference in
population age. This indicates that the light-curve fitters used by the SNe Ia
community are not quite capable of correcting for the population age effect,
which would inevitably cause a serious systematic bias with look-back time.
Notably, taken at face values, a significant fraction of the Hubble residual
used in the discovery of the dark energy appears to be affected by the
luminosity evolution. We argue, therefore, that this systematic bias must be
considered in detail in SN cosmology before proceeding to the details of the
dark energy.
The most direct and strongest evidence for the presence of dark energy is
provided by the measurement of galaxy distances using type Ia supernovae (SNe
Ia). This result is based on the assumption that the corrected brightness of SN
Ia through the empirical standardization would not evolve with look-back time.
Recent studies have shown, however, that the standardized brightness of SN Ia
is correlated with host morphology, host mass, and local star formation rate,
suggesting a possible correlation with stellar population property. In order to
understand the origin of these correlations, we have continued our
spectroscopic observations to cover most of the reported nearby early-type host
galaxies. From high-quality (signal-to-noise ratio ~175) spectra, we obtained
the most direct and reliable estimates of population age and metallicity for
these host galaxies. We find a significant correlation between SN luminosity
(after the standardization) and stellar population age at a 99.5% confidence
level. As such, this is the most direct and stringent test ever made for the
luminosity evolution of SN Ia. Based on this result, we further show that the
previously reported correlations with host morphology, host mass, and local
star formation rate are most likely originated from the difference in
population age. This indicates that the light-curve fitters used by the SNe Ia
community are not quite capable of correcting for the population age effect,
which would inevitably cause a serious systematic bias with look-back time.
Notably, taken at face values, a significant fraction of the Hubble residual
used in the discovery of the dark energy appears to be affected by the
luminosity evolution. We argue, therefore, that this systematic bias must be
considered in detail in SN cosmology before proceeding to the details of the
dark energy.
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