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

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.