The Mean Type Ia Supernova Spectrum Over the Past 9 Gigayears

TL;DR

This study investigates the evolution of Type Ia Supernova spectra over 9 gigayears, revealing a decrease in intermediate-mass element features with redshift and supporting their use as standard candles.

Contribution

It provides new evidence of spectral evolution in SNe Ia over cosmic time and models this evolution with a stretch distribution, enhancing understanding of their use in cosmology.

Findings

Decreased IME features at higher redshift

Spectral evolution consistent with stretch distribution models

Colors explained by stretch variations

Abstract

We examine the possibility of evolution with redshift in the mean rest-frame ultraviolet (UV; <4500A) spectrum of Type Ia Supernovae (SNe Ia) sampling the redshift range 0<z<1.3. We find new evidence for a decrease with redshift in the strength of intermediate-mass element (IME) features, particularly Si II and to a lesser extent Ca II "H&K" and Mg II blends, indicating lower IME abundances in the higher redshift SNe. A larger fraction of luminous, wider light-curve width (higher "stretch") SNe Ia are expected at higher redshift than locally, so we compare our observed spectral evolution with that predicted by a redshift-evolving stretch distribution (Howell et al. 2007) coupled with a stretch-dependent SN Ia spectrum. We show that the sense of the spectral evolution can be reproduced by this simple model, though the highest redshift events seem additionally deficient in Si and Ca. We also examine the mean SN Ia UV-optical colors as a function of redshift, thought to be sensitive to variations in progenitor composition. We find that the expected stretch variations are sufficient to explain the differences, although improved data at z~0 will enable more precise tests. Thus, to the extent possible with the available datasets, our results support the continued use of SNe Ia as standardized candles.