ZTF SN Ia DR2: Environmental dependencies of stretch and luminosity for a volume-limited sample of 1000 type Ia supernovae

¹ Univ Lyon, Univ Claude Bernard Lyon 1, CNRS, IP2I Lyon/IN2P3, UMR 5822, F-69622 Villeurbanne, France
² Department of Physics, Lancaster University, Lancs LA1 4YB, UK
³ School of Physics, Trinity College Dublin, College Green, Dublin 2, Ireland
⁴ Oskar Klein Centre, Department of Physics, Stockholm University, SE-10691 Stockholm, Sweden
⁵ Institut für Physik, Humboldt Universität zu Berlin, Newtonstr 15, 12101 Berlin, Germany
⁶ National Research Council of Canada, Herzberg Astronomy & Astrophysics Research Centre, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
⁷ Université Clermont Auvergne, CNRS/IN2P3, LPCA, F-63000 Clermont-Ferrand, France
⁸ Sorbonne Université, CNRS/IN2P3, LPNHE, F-75005 Paris, France
⁹ Aix Marseille Université, CNRS/IN2P3, CPPM, Marseille, France
¹⁰ Department of Physics, Duke University, Durham, NC 27708, USA
¹¹ Institute of Astronomy and Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹² Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain
¹³ Institut d’Estudis Espacials de Catalunya (IEEC), E-08034 Barcelona, Spain
¹⁴ Deutsches Elektronen Synchrotron DESY, Platanenallee 6, 15738 Zeuthen, Germany
¹⁵ Lawrence Berkeley National Laboratory, 1 Cyclotron Road, MS, 50B-4206 Berkeley, CA 94720, USA
¹⁶ Department of Astronomy, University of California, Berkeley, 501 Campbell Hall, Berkeley, CA 94720, USA
¹⁷ Oskar Klein Centre, Department of Astronomy, Stockholm University, SE-10691 Stockholm, Sweden
¹⁸ Nordic Optical Telescope, Rambla José Ana Fernández Pérez 7, ES-38711 Breña Baja, Spain
¹⁹ Caltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, USA
²⁰ DIRAC Institute, Department of Astronomy, University of Washington, 3910 15th Avenue NE, Seattle, WA 98195, USA
²¹ Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
²² IPAC, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA

^⋆ Corresponding author; m.ginolin@ip2i.in2p3.fr

Received: 15 April 2024
Accepted: 31 January 2025

Abstract

Context. Type Ia supernova (SN Ia) cosmology studies will soon be dominated by systematic, uncertainties, rather than statistical ones. Thus, it is crucial to understand the unknown phenomena potentially affecting their luminosity that may remain, such as astrophysical biases. For their accurate application in such studies, SN Ia magnitudes need to be standardised; namely, they must be corrected for their correlation with the light-curve width and colour.

Aims. Here, we investigate how the standardisation procedure used to reduce the scatter of SN Ia luminosities is affected by their environment. Our aim is to reduce scatter and improve the standardisation process.

Methods. We first studied the SN Ia stretch distribution, as well as its dependence on environment, as characterised by local and global (g − z) colour and stellar mass. We then looked at the standardisation parameter, α, which accounts for the correlation between residuals and stretch, along with its environment dependency and linearity. Finally, we computed the magnitude offsets between SNe in different astrophysical environments after the colour and stretch standardisations (i.e. steps). This analysis has been made possible thanks to the unprecedented statistics of the volume-limited Zwicky Transient Facility (ZTF) SN Ia DR2 sample.

Results. The stretch distribution exhibits a bimodal behaviour, as previously found in the literature. However, we find the distribution to be dependent on environment. Specifically, the mean stretch modes decrease with host stellar mass, at a 9.2σ significance. We demonstrate, at the 13.4σ level, that the stretch-magnitude relation is non-linear, challenging the usual linear stretch-residuals relation currently used in cosmological analyses. In fitting for a broken-α model, we did indeed find two different slopes between stretch regimes (x₁ ≶ x₁⁰ with x₁⁰ = −0.48 ± 0.08): α_low = 0.271 ± 0.011 and α_high = 0.083 ± 0.009, comprising a difference of Δα = −0.188 ± 0.014. As the relative proportion of SNe Ia in the high-stretch and low-stretch modes evolves with redshift and environment, this implies that a single-fitted α also evolves with the redshift and environment. Concerning the environmental magnitude offset γ, we find it to be greater than 0.12 mag, regardless of the considered environmental tracer used (local or global colour and stellar mass), all measured at the ≥5σ level. When accounting for the non-linearity of the stretch, these steps increase to ∼0.17 mag, measured with a precision of 0.01 mag. Such strong results highlight the importance of using a large volume-limited dataset to probe the underlying SN Ia-host correlations.