ZTF SN Ia DR2: Colour standardisation of type Ia supernovae and its dependence on the environment

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

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

Received: 31 May 2024
Accepted: 10 January 2025

Abstract

Context. As type Ia supernova cosmology transitions from a statistics-dominated to a systematics-dominated era, it is crucial to understand the remaining unexplained uncertainties that affect their luminosity, such as those stemming from astrophysical biases. Type Ia supernovae are standardisable candles whose absolute magnitude reaches a scatter of typically 0.15 mag when empirical correlations with their light-curve stretch and colour and with their environmental properties are accounted for.

Aims. We investigate the dependence of the standardisation process of type Ia supernovae on the astrophysical environment to ultimately reduce their scatter in magnitude. We focus on colour standardisation.

Methods. We used the volume-limited ZTF SN Ia DR2 sample, which offers unprecedented statistics for the low-redshift (z < 0.06) range. We first studied the colour distribution with a focus on the effects of dust to then select a dustless subsample of objects that originated in environments with a low stellar mass and in the outskirts of their host galaxies. We then examined the colour-residual relation and its associated parameter β. Finally, we investigated the colour dependence of the environment-dependent magnitude offsets (steps) to separate their intrinsic and extrinsic components.

Results. Our sample of nearly 1000 supernovae probes the red tail of the colour distribution up to c = 0.8. The dustless sample exhibits a significantly shorter red tail (4.3σ) than the whole sample, but the distributions around c ∼ 0 are similar for both samples. This suggests that the reddening above c ≥ 0.2 is dominated by interstellar dust absorption of the host and that the remaining colour scatter has an intrinsic origin. The colour-residual relation is linear with light-curve colour. We found indications of a potential evolution of β with the stellar host mass, with β ∼ 3.6 for low-mass galaxies, compared to β = 3.05 ± 0.06 for the full sample. Finally, in contrast to recent claims from the literature, we found no evolution of steps as a function of light-curve colour. This suggests that dust may not be the dominating mechanism for the dependence on the environment of the magnitude of type Ia supernovae.