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 within our simplified modelling is linked to larger ejecta masses providing longer diffusion times, providing a physical basis for the brighter-slower relation. SN~Ia in low-mass hosts (${\log }_{10}(M_{\ast }/M_{\odot })<10$) produce $12\%$ more ${}^{56}$Ni than those in high-mass hosts ($\hat{I}”M_{\mathrm{Ni}}=0.13M_{\odot }$), linking the host-galaxy mass step to ejecta properties and hinting at metallicity or age-dependent burning efficiencies. This suggests that standardisation based on physical parameters may remove the mass-step. SN~1991T-like events show higher ejecta masses (median $1.64M_{\odot }$ vs. $1.38M_{\odot }$ for normals) and produce $30\%$ more ${}^{56}$Ni, with $84\%$ having super-Chandrasekhar masses. Through Hierarchical modelling of $902$ SNe ($z\le 0.06$), we find thermonuclear supernovae can be well described by a Gaussian distribution in ejecta mass and nickel mass with $\hat{I}{¼}_{\mathrm{ej}}=1.26\pm 0.01M_{\odot }$ ($\ddot{I}{ƒ}_{\mathrm{ej}}=0.33\pm 0.01M_{\odot }$) and $\hat{I}{¼}_{\mathrm{Ni}}=0.64\pm 0.06M_{\odot }$ ($\ddot{I}{ƒ}_{\mathrm{Ni}}=0.42\pm 0.02M_{\odot }$), respectively. This leads to inferred fractions of $43\pm 2\%$ sub-$M_{\mathrm{Ch}}$ ($<1.2M_{\odot }$), $34\pm 1\%$ near-$M_{\mathrm{Ch}}$ ($1.2$--$1.5M_{\odot }$), and $24\pm 2\%$ super-$M_{\mathrm{Ch}}$ ($>1.5M_{\odot }$) events. This work provides a step towards holistic physical characterization of the local SN~Ia population, reinforcing the physical basis of SN~Ia standardization while quantifying diversity and environmental dependencies critical for understanding progenitor physics and mitigating systematics in precision cosmology.

Submitted. 24 pages 21 figures