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

Abstract

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 (${\log }_{10}(M_{\ast }/M_{\odot })<10$) produce $\approx 12\%$ more ${}^{56}$Ni 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 ($z\le 0.06$) gives Gaussian population distributions with ${\mu }_{\mathrm{ej}}=1.26\pm 0.01M_{\odot }$ (${\sigma }_{\mathrm{ej}}=0.33\pm 0.01M_{\odot }$) and ${\mu }_{\mathrm{Ni}}=0.64\pm 0.06M_{\odot }$ (${\sigma }_{\mathrm{Ni}}=0.42\pm 0.02M_{\odot }$). 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.

Accepted in MNRAS. 28 pages 21 figures

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