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

TL;DR

This study models 2,205 Type Ia supernovae light curves to understand their physical properties, host galaxy influences, and implications for cosmology, revealing correlations between nickel mass, ejecta mass, and host galaxy mass.

Contribution

It introduces a physical model linking supernova light curve parameters to ejecta and nickel masses, and explores environmental effects on supernova diversity and standardization.

Findings

Strong correlation between nickel mass and SALT2 stretch.

Low-mass host galaxies produce more nickel.

A significant fraction of supernovae have super-Chandrasekhar masses.

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 ($\log_{10}(M_*/M_\odot) < 10$) produce $12\%$ more $^{56}$Ni than those in high-mass hosts ($\Delta M_{\rm Ni} = 0.13~M_\odot$), 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 $1.64~M_\odot$ vs. $1.38~M_\odot$ for normals) and produce $30\%$ more $^{56}$Ni, with $84\%$ having super-Chandrasekhar masses. Through Hierarchical modelling of $902$ SNe ($z \leq 0.06$), we find thermonuclear supernovae can be well described by a Gaussian distribution in ejecta mass and nickel mass with $\mu_{\rm ej} = 1.26 \pm 0.01~M_\odot$ ($\sigma_{\rm ej} = 0.33 \pm 0.01~M_\odot$) and $\mu_{\rm Ni} = 0.64 \pm 0.06~M_\odot$ ($\sigma_{\rm Ni} = 0.42 \pm 0.02~M_\odot$), respectively. This leads to inferred fractions of $43 \pm 2\%$ sub-$M_{\rm Ch}$ ($<1.2~M_\odot$), $34 \pm 1\%$ near-$M_{\rm Ch}$ ($1.2$--$1.5~M_\odot$), and $24 \pm 2\%$ super-$M_{\rm Ch}$ ($>1.5~M_\odot$) 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.