ZTF SN Ia DR2: The spectral diversity of Type Ia supernovae in a volume-limited sample

TL;DR

This paper analyzes the spectral diversity of over 480 Type Ia supernovae from the ZTF DR2, exploring spectral features, classifications, and modeling to understand their physical properties and variations.

Contribution

It introduces a new subclass of underluminous SNe Ia (`04gs-like'), investigates spectral feature relations, and assesses spectral template matching accuracy.

Findings

Identified a new `04gs-like' SN Ia subclass with unique spectral features.

Showed that temperature alone cannot explain `04gs-like' spectra, indicating abundance variations.

Demonstrated spectral continuity across different SN Ia subclasses, supporting unified explosion models.

Abstract

More than 3000 spectroscopically confirmed Type Ia supernovae (SNe Ia) are presented in the Zwicky Transient Facility SN Ia Data Release 2 (ZTF DR2). In this paper, we detail the spectral properties of 482 SNe Ia near maximum light, up to a redshift limit of $z$ $\leq$ 0.06. We measure the velocities and pseudo-equivalent widths (pEW) of key spectral features (Si II $\lambda$5972 and Si II $\lambda$6355) and investigate the relation between the properties of the spectral features and the photometric properties from the SALT2 light-curve parameters as a function of spectroscopic sub-class. We discuss the non-negligible impact of host galaxy contamination on SN Ia spectral classifications, as well as investigate the accuracy of spectral template matching of the ZTF DR2 sample. We define a new subclass of underluminous SNe Ia (`04gs-like') that lie spectroscopically between normal SNe Ia and transitional 86G-like SNe Ia (stronger Si II $\lambda$5972 than normal SNe Ia but significantly weaker Ti II features than `86G-like' SNe). We model these `04gs-like' SN Ia spectra using the radiative-transfer spectral synthesis code tardis and show that cooler temperatures alone are unable to explain their spectra; some changes in elemental abundances are also required. However, the broad continuity in spectral properties seen from bright (`91T-like') to faint normal SN Ia, including the transitional and 91bg-like SNe Ia, suggests that variations within a single explosion model may be able to explain their behaviour.