Early-type Host Galaxies of Type Ia Supernovae. II. Evidence for Luminosity Evolution in Supernova Cosmology

TL;DR

This study provides strong evidence that the luminosity of Type Ia supernovae evolves with stellar population age, which could introduce systematic biases in cosmological measurements of dark energy.

Contribution

It offers the most direct measurement of SN Ia luminosity evolution linked to host galaxy stellar age, challenging current standardization methods.

Findings

Significant correlation between SN luminosity and host galaxy age.

Host morphology, mass, and SFR effects are driven by stellar age differences.

Luminosity evolution may bias dark energy measurements.

Abstract

The most direct and strongest evidence for the presence of dark energy is provided by the measurement of galaxy distances using 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 (SFR), suggesting a possible correlation with stellar population property. 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 SFR 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, most of the Hubble residual used in the discovery of the dark energy appears to be affected by the luminosity evolution.