Diversity of supernovae Ia determined using equivalent widths of Si II 4000

TL;DR

This study analyzes the spectral diversity of Type Ia supernovae using Si II 4000 equivalent widths, revealing consistent sub-classes across redshifts and enabling easier high-redshift classification.

Contribution

The paper introduces a wavelet-based spectral measurement method for classifying SNe Ia sub-types, applicable at high redshift, and confirms the stability of spectral features over cosmic time.

Findings

No evolution of Si II 4000 equivalent width with redshift.

Three supernova sub-classes identified using spectral and photometric data.

High-redshift SNe Ia follow similar grouping as nearby ones.

Abstract

Spectroscopic and photometric properties of low and high-z supernovae Ia (SNe Ia) have been analyzed in order to achieve a better understanding of their diversity and to identify possible SN Ia sub-types. We use wavelet transformed spectra in which one can easily measure spectral features. We investigate the \ion{Si}{II} 4000 equivalent width ($EW_w\lbrace\ion{Si}{II}\rbrace$). The ability and, especially, the ease in extending the method to SNe at high-$z$ is demonstrated. We applied the method to 110 SNe Ia and found correlations between $EW_w\lbrace\ion{Si}{II}\rbrace$ and parameters related to the light-curve shape for 88 supernovae with available photometry. No evidence for evolution of $EW_w\lbrace\ion{Si}{II}\rbrace$ with redshift is seen. Three sub-classes of SNe Ia were confirmed using an independent cluster analysis with only light-curve shape, colour, and $EW_w\lbrace\ion{Si}{II}\rbrace$. SNe from high-$z$ samples seem to follow a similar grouping to nearby objects. The $EW_w\lbrace\ion{Si}{II}\rbrace$ value measured on a single spectrum may point towards SN Ia sub-classification, avoiding the need for expansion velocity gradient calculations.