ZTF SN Ia DR2: Improved SN Ia colors through expanded dimensionality with SALT3+

¹ The Oskar Klein Centre, Department of Physics, Stockholm University, SE-106 91 Stockholm, Sweden
² Institute for Astronomy, University of Hawai’i, 640 N. Aohoku Pl., Hilo, HI 96720, USA
³ Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA
⁴ Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL 60637, USA
⁵ School of Physics, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland
⁶ Institute of Astronomy, Madingley Rd, Cambridge CB3 0HA, United Kingdom
⁷ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, E-08193 Barcelona, Spain
⁸ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels (Barcelona), Spain
⁹ Univ. Lyon, Univ. Claude Bernard Lyon 1, CNRS, IP2I Lyon/IN2P3, UMR 5822, F-69622 Villeurbanne, France
¹⁰ Department of Physics, Lancaster University, Lancs LA1 4YB, UK
¹¹ Lawrence Berkeley National Laboratory, 1 Cyclotron Road MS 50B-4206, Berkeley, CA 94720, USA
¹² Department of Astronomy, University of California, Berkeley, 501 Campbell Hall, Berkeley, CA 94720, USA
¹³ Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
¹⁴ Université Clermont Auvergne, CNRS/IN2P3, LPCA, F-63000 Clermont-Ferrand, France
¹⁵ The Oskar Klein Centre, Department of Astronomy, Stockholm University, SE-106 91 Stockholm, Sweden
¹⁶ Nordic Optical Telescope, Rambla José Ana Fernández Pérez 7, ES-38711 Breña Baja, Spain
¹⁷ IPAC, California Institute of Technology, 1200 E. California Boulevard, Pasadena CA 91125, USA
¹⁸ Caltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, USA
¹⁹ Aix Marseille Université, CNRS/IN2P3, CPPM, Marseille, France

^⋆ Corresponding author: darcy.kenworthy@fysik.su.se

Received: 11 October 2024
Accepted: 7 March 2025

Abstract

Context. Type Ia supernovae (SNe Ia) are a key probe in modern cosmology, as they can be used to measure luminosity distances at gigaparsec scales. Models of their light curves are used to project heterogeneous observed data onto a common basis for analysis.

Aims. The SALT model currently used for SN Ia cosmology describes SNe as having two sources of variability, accounted for by a color parameter c, and a “stretch” parameter x₁. We extend the model to include an additional parameter we label x₂, to investigate the cosmological impact of currently unaddressed light-curve variability.

Methods. We constructed a new SALT model, that we dub “SALT3+”. This model was trained by an improved version of the SALTshaker code, using training data combining a selection of the second data release of cosmological SNe Ia from the Zwicky Transient Facility and the existing SALT3 training compilation.

Results. We find additional, coherent variability in supernova light curves beyond SALT3. Most of this variation can be described as phase-dependent variation in g − r and r − i color curves, correlated with a boost in the height of the secondary maximum in i-band. These behaviors correlate with spectral differences, particularly in line velocity. We find that fits with the existing SALT3 model tend to address this excess variation with the color parameter, leading to less informative measurements of supernova color. We find that neglecting the new parameter in light-curve fits leads to a trend in Hubble residuals with x₂ of 0.039 ± 0.005 mag, representing a potential systematic uncertainty. However, we find no evidence of a bias in current cosmological measurements.

Conclusions. We conclude that extended SN Ia light-curve models promise mild improvement in the accuracy of color measurements, and corresponding cosmological precision. However, models with more parameters are unlikely to substantially affect current cosmological results.