On the origin of the environmental dependence of SN Ia magnitudes: A BayeSN view of the ZTF SN Ia DR2

Astrophysical variabilities of Type Ia supernovae (SNe Ia), such as their link with their birth environment, are now one of the leading sources of systematic uncertainties on the measurement of the dark energy equation-of-state parameter w. Population studies of SNe Ia, using large samples, give precious insights into these variabilities. We analyse a volume-limited subsample of 932 SNe from the ZTF SN Ia DR2 with BayeSN, a hierarchical Bayesian model for SN Ia SEDs. We investigate the distributions of SN Ia light curve parameters and their link with SN environment. Using a new training of BayeSN released in a companion paper, we find a smaller scatter of Hubble residuals compared to SALT. We then investigate the magnitude step, which accounts for the correlation between SN Ia standardised absolute magnitude and host environments. We find a posteriori steps of 0.103 plus or minus0.010 mag (a 10.1σ difference from 0) when using global stellar mass as an environmental proxy, and 0.086 plus or minus0.010 mag (8.3σ) when using local colour, in accordance with steps computed using SALT light curve fits. This confirms that the large step seen in the ZTF SN Ia DR2 data was not due to the SALT fit or the associated standardisation process. We then investigate the origin of the step, using a BayeSN model which accounts for both an intrinsic magnitude step and differing dust properties with the SN environment. We find a 0.103 plus or minus0.018 mag (5.6σ) step in global mass and a 0.085 plus or minus0.019 mag (4.5σ) step in local colour. The means of the R V distribution are similar between different host environments, with Δ E(R V) at most0.2 across all environment proxies, with significances ranging from 0.6σ to 1.2σ. This is a strong signal of the existence of an intrinsic dependence of SN Ia absolute magnitude on environment.