Union3.1: Self-consistent Measurements of Host Galaxy Properties for 2000 Type Ia Supernovae

TL;DR

This study self-consistently measures host galaxy properties for 2000 Type Ia supernovae, improving distance estimates and cosmological parameter constraints by correcting previous biases and uncertainties.

Contribution

It introduces a uniform method for deriving host galaxy properties across a large supernova sample, reducing systematic biases and refining cosmological measurements.

Findings

Host galaxy mass estimates were corrected, reducing discrepancies between studies.

Uncertainties on supernova standardization parameters decreased significantly.

Cosmological parameters showed slight shifts, with improved evidence against a cosmological constant.

Abstract

The determination of distances using time-series photometry of Type Ia supernovae (SNe Ia) relies on a ~5% empirical correction related to the properties of their host galaxies, e.g., global stellar mass. It is therefore crucial for unbiased cosmology inference that host galaxy properties be self-consistently determined across the full range of redshifts probed, which we undertake in this study for approximately 2000 SNe in the Union3 compilation (now Union3.1). We use aperture-matched, homogeneously-reduced, optical-infrared photometry from the DESI Legacy Imaging Surveys to derive global galaxy properties using the stellar population synthesis and SED-fitting code Prospector. We find that the host masses of $z<0.10$ SNe in Union3 were, on average, overestimated relative to the rest of the sample, while the opposite was true for $z<0.15$ SNe in Pantheon+. After correction, the two studies' average distance modulus estimated for low-redshift SNe, previously $>0.03$ mag discrepant, come into 0.01 mag agreement. We then update the UNITY SN analysis and find that the uncertainties on all standardization parameters shrink to 0.6-0.9x their previous sizes. For flat-$\Lambda$CDM, we find $\Omega_m=0.344^{+0.026}_{-0.025}$, a -0.3$\sigma$ shift from Union3. We then combine with measurements of Baryon Acoustic Oscillations (BAO) and the Cosmic Microwave Background (CMB) exactly as done by DESI DR2 and find $w_0=-0.719\pm0.084$, $w_a=-0.95^{+0.29}_{-0.26}$, corresponding to 3.4$\sigma$ evidence against a cosmological constant (down from 3.8$\sigma$). We also update the DESI combined probe analysis using our correction to Pantheon+ and the recent DES-SN5YR Dovekie recalibration, finding $3.2\sigma$ (up from 2.8$\sigma$) and 3.4$\sigma$ (down from 4.2$\sigma$) evidence against a cosmological constant in the $w_0w_a$ plane, altogether marking a significantly improved consistency across SN analyses.