Supernova Legacy Survey: using spectral signatures to improve Type Ia supernovae as distance indicators




Optical long-slit spectroscopy at the Gemini-North telescope using the Gemini Multi-Object Spectrograph (GMOS) was used to classify targets from the Supernova Legacy Survey (SNLS) from 2005 July and 2006 May–2008 May. 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 ≤z≤ 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 – Ca ii H&K, Si ii and Mg ii– 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 Ca ii H&K and Mg ii features. Systematically lower EW Si ii 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 Si ii and photometric light-curve stretch.