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

TL;DR

Union3.1 provides self-consistent host-galaxy mass estimates for ~2000 Type Ia SNe by leveraging homogeneous DESI Legacy Imaging Survey photometry and Prospector SED fitting, placing most SNe on a single mass scale. Correcting prior mass offsets between Union3 and Pantheon+ reduces biases in distance moduli and tightens SN standardization uncertainties by roughly 0.6–0.9×. In a flat-$Λ$CDM and with DESI BAO/CMB priors, the analysis finds $Ω_m o 0.344^{+0.026}_{-0.025}$, and when combined with BAO/CMB data yields evolving dark-energy constraints with $w_0 o -0.719$ and $w_a o -0.95$, i.e., ~3.4σ from a cosmological constant. Pantheon+ is similarly updated and the joint constraints become more consistent across SN analyses, reducing the previously reported tension and strengthening the case for dynamics in the dark-energy equation of state. Overall, homogeneous host-mass calibration emerges as a critical systematic, improving cosmological inferences and cross-survey consistency for SN cosmology.

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-$Λ$CDM, we find $Ω_m=0.344^{+0.026}_{-0.025}$, a -0.3$σ$ 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$σ$ evidence against a cosmological constant (down from 3.8$σ$). We also update the DESI combined probe analysis using our correction to Pantheon+ and the recent DES-SN5YR Dovekie recalibration, finding $3.2σ$ (up from 2.8$σ$) and 3.4$σ$ (down from 4.2$σ$) evidence against a cosmological constant in the $w_0w_a$ plane, altogether marking a significantly improved consistency across SN analyses.

Figures (25)

• Figure 1: Positions on the sky for the 682 low-redshift ($z < 0.15$) SNe in Union3.1 that passed UNITY cosmology cuts (circle markers). 559 (82.0%) have been assigned a host galaxy contained in the Legacy Surveys (LS) DR10 Tractor catalog (filled circles). The majority of the low-redshift SNe for which we did not find a counterpart (open circles) were not covered by the BASS+MzLS (red-shaded polygon) or the DECaLS (blue-shaded polygon) DR10 footprints. In the intermediate redshift bin ($0.15 \lesssim z \lesssim 0.90$), 968 of 1335 (72.5%) SNe were assigned to a source in the LS DR10 catalog. The regions of the sky targeted by those surveys are marked (small unfilled polygons).
• Figure 2: Per-survey breakdown of the number of Union3.1 SNe whose host mass comes from the new Legacy Survey Tractor + Prospector (LSTP) analysis (blue patches), another published source (red patches), or for which we have no host information (clear/white patches). In \ref{['fig:massComp_summary']}, we will directly compare the LSTP mass estimates with these Other Sources.
• Figure 3: Comparison of the LSTP stellar masses determined in this study with other studies or compilations. Filled circles represent the median mass offset between LSTP and a published mass source that we use to fill gaps in the LSTP masses. X markers represent the mean, while error bars represent the 68th percentile. The error on the mean of each offset is smaller than the size of the marker. The mix of mass sources we use in Union3.1 to supplement the LSTP masses demonstrate generally good agreement at the $\leq 0.25$ dex level. Notably, the Union3 and Pantheon+ compilations exhibit large, redshift dependent offsets of 0.6 to 0.8 dex between their low-redshift and high-redshift samples. The DES-SN5YR masses are systematically offset from LSTP, but are internally self-consistent across redshifts.
• Figure 4: Distributions of host masses for low-redshift $(z<0.15)$ surveys in Union3.1. The bins have width 0.35 dex. The gray shaded curve is a Gaussian KDE representation of the host mass distribution for all SNe in Union3.1. Two summary statistics are included: (1) the mean mass and a bootstrapped error on the mean, (2) the 90th percentile interval of masses. The mass distributions of galaxy-targeted surveys such as CfA, CSP, Krisciunas, and LOSS peak at large values between 10.8 and 11.0 dex, while NB99, LSQ, CNIa02, and Foundation peak at much lower values between 10.2 and 10.4 dex, because those surveys observed SNe discovered by untargeted searches such as ASAS-SN, LSQ, and PSST.
• Figure 5: Same as \ref{['fig:massHists_lowz']}, but for the $z \geq 0.15$ surveys in Union3.1. HZT (T03), SCP (K03 and N09), SCP (A10), and MCT (R18) are not pictured here because directly determined host masses do not exist for those samples. SNe in the SuzukiRubin sample come from a galaxy cluster search and are thus found exclusively in massive hosts.