Supernova Legacy Survey: Using Spectral Signatures To Improve Type Ia Supernovae As Distance Indicators

TL;DR

This study uses spectral signatures from supernovae to enhance the accuracy of Type Ia supernovae as cosmic distance indicators, analyzing spectral features across different redshifts to understand population changes.

Contribution

It introduces a spectral analysis method focusing on specific features to improve supernovae standardization and investigates spectral evolution with redshift.

Findings

No evolution in CaII H&K and MgII features with redshift.

Lower EW SiII at high redshift explained by population demographics.

Correlation between EW SiII and lightcurve stretch observed.

Abstract

GMOS optical long-slit spectroscopy at the Gemini-North telescope was used to classify targets from the Supernova Legacy Survey (SNLS) from July 2005 and May 2006 - May 2008. During this time, 95 objects were observed. Where possible the objects' redshifts (z) were measured from narrow emission or absorption features in the host galaxy spectrum, otherwise they were measured from the broader supernova features. We present spectra of 68 confirmed or probable SNe Ia from SNLS with redshifts in the range 0.17 \leq z \leq 1.02. In combination with earlier SNLS Gemini and VLT spectra, we used these new observations to measure pseudo-equivalent widths (EWs) of three spectral features - CaII H&K, SiII and MgII - in 144 objects and compared them to the EWs of low-redshift SNe Ia from a sample drawn from the literature. No signs of changes with z are seen for the CaII H&K and MgII features. Systematically lower EW SiII is seen at high redshift, but this can be explained by a change in demographics of the SNe Ia population within a two-component model combined with an observed correlation between EW SiII and photometric lightcurve stretch.