Lightcurve Modelling of 2,205 ZTF DR2 Type Ia Supernovae: Implications for SN Ia Physics and Cosmology

We fit the multi-band light curves of 2,205 Type Ia supernovae (SNe Ia) from the Zwicky Transient Facility DR2 with a one-zone radioactive decay model with a phenomenological addition to include Fe recombination physics. We find a strong correlation between inferred nickel mass and SALT2 stretch, which our simplified modelling links to longer diffusion times in more massive ejecta, offering a physical basis for the brighter-slower relation. SNe Ia in low-mass hosts (log10(M*/M⊙) < 10) produce ≈12% more 56Ni than those in high-mass hosts, linking the host-galaxy mass step to ejecta properties and hinting at metallicity or age-dependent burning efficiencies. A pseudo-bolometric comparison provides lower limits on the nickel masses, highlighting their sensitivity to SED-level assumptions. Injection-and-recovery tests with realistic ZTF sampling and the same model recover the nickel scale but show significant sensitivity to distance and opacity assumptions; individual-event point estimates are therefore model-dependent. Accounting for selection biases and broad individual-event posteriors, hierarchical modelling of 902 SNe (z ≤ 0.06) gives Gaussian population distributions with μej = 1.26 ± 0.01 M⊙ (σej = 0.33 ± 0.01 M⊙) and μNi = 0.64 ± 0.06 M⊙ (σNi = 0.42 ± 0.02 M⊙). This work provides a step towards physical characterization of the local SN Ia population while quantifying diversity and environmental dependencies relevant to progenitor physics and precision cosmology.