The Twins Embedding of Type Ia Supernovae II: Improving Cosmological Distance Estimates

TL;DR

This paper demonstrates how spectra-based manifold learning, called the Twins Embedding, can improve the standardization of Type Ia supernovae for more accurate cosmological distance measurements, reducing biases and host galaxy correlations.

Contribution

It introduces the Twins Embedding as a novel spectral parameterization that enhances supernova standardization beyond traditional light curve methods.

Findings

Achieves RMS of 0.101 mag in supernova standardization with a single spectrum.

Reduces bias in distance estimates for peculiar SNe Ia.

Decreases host galaxy correlation in supernova distance measurements.

Abstract

We show how spectra of Type Ia supernovae (SNe Ia) at maximum light can be used to improve cosmological distance estimates. In a companion article, we used manifold learning to build a three-dimensional parameterization of the intrinsic diversity of SNe Ia at maximum light that we call the "Twins Embedding". In this article, we discuss how the Twins Embedding can be used to improve the standardization of SNe Ia. With a single spectrophotometrically-calibrated spectrum near maximum light, we can standardize our sample of SNe Ia with an RMS of $0.101 \pm 0.007$ mag, which corresponds to $0.084 \pm 0.009$ mag if peculiar velocity contributions are removed and $0.073 \pm 0.008$ mag if a larger reference sample were obtained. Our techniques can standardize the full range of SNe Ia, including those typically labeled as peculiar and often rejected from other analyses. We find that traditional light curve width + color standardization such as SALT2 is not sufficient. The Twins Embedding identifies a subset of SNe Ia including but not limited to 91T-like SNe Ia whose SALT2 distance estimates are biased by $0.229 \pm 0.045$ mag. Standardization using the Twins Embedding also significantly decreases host-galaxy correlations. We recover a host mass step of $0.040 \pm 0.020$ mag compared to $0.092 \pm 0.024$ mag for SALT2 standardization on the same sample of SNe Ia. These biases in traditional standardization methods could significantly impact future cosmology analyses if not properly taken into account.