Supernovae Unite: Host-Galaxy Mass Measurements of Type Ia Supernovae and Their Impact on Cosmology

J. Lee; R. Camilleri; T. M. Davis; D. Rubin; K. Bechtol; L. Galbany; M. Sako; M. Sullivan; T. E. Müller-Bravo; D. Scolnic; M. Vincenzi; D. Brout; C. Lidman; A. Möller; P. Shah; M. Acevedo; P. Armstrong; B. A. Bassett; R. C. Chen; H. T. Diehl; J. Frieman; K. Grech; B. Popovic; B. O. Sánchez; B. E. Tucker

paper

Supernovae Unite: Host-Galaxy Mass Measurements of Type Ia Supernovae and Their Impact on Cosmology

arXiv:2609.05321

Abstract

Current consensus suggests that Type Ia supernova (SN Ia) brightnesses post light-curve standardization correlate with their host-galaxy stellar masses, which must be accounted for to obtain accurate cosmological constraints. For example, Vincenzi et al. (2025) showed that different host-galaxy stellar mass measurements for the same dataset produce redshift-dependent differences of order $\sim 0.01$ mag, large enough to appreciably shift cosmological constraints. We present internally consistent host-galaxy stellar masses remeasured using aperture photometry and spectral energy distribution (SED) fitting for SN-Unite, which combines the spectroscopic Pantheon+ and the photometric Dark Energy Survey five-year (DES-SN5YR) samples into the largest SN Ia cosmology sample to date, with 2884 likely SNe Ia. We find that photometry and SED fitting choices shift SN-Unite Flat $w$ CDM parameters well below statistical uncertainties. Our stellar masses differ from the Pantheon+ data release partly due to a redshift-dependent internal inconsistency within Pantheon+, while remaining largely consistent with the DES-SN5YR (DES-Dovekie) data release. When the Pantheon+ subsample of SN-Unite is combined with Baryon Acoustic Oscillations (BAO) and Cosmic Microwave Background (CMB) measurements, the significance for time-evolving dark energy increases from 3.4 $σ$ to 4.0 $σ$ based on the maximum a posteriori when our newly derived host-galaxy stellar masses replace the Pantheon+ data-release host-galaxy stellar masses, while DES-Dovekie remains virtually unchanged, consistent with the findings of Hoyt et al. (2026). By remeasuring the host-galaxy stellar masses using a consistent framework throughout the whole sample, we improve the robustness of the SN-Unite cosmological constraints against systematic differences in host-galaxy stellar mass measurements.

25 pages, 10 figures. Companion paper to Supernovae Unite: Combining Pantheon+ and DES-SN5YR. The host-galaxy stellar mass measurements and photometry will be publicly available upon acceptance of the paper. V2: For our Youtube Cosmology Talks, see https://www.youtube.com/watch?v=KT-j65gRnOQ. Removed inactive datalinks and fixed MAP significance. Submitted to the Astrophysical Journal