The ESO/VLT 3rd year Type Ia supernova data set from the Supernova Legacy Survey

TL;DR

This paper presents a large, homogeneous set of 124 Type Ia supernova spectra from the first three years of the SNLS, used to analyze spectral evolution and test for redshift-dependent differences in supernova properties.

Contribution

It provides the largest spectral dataset of SNeIa in the redshift range 0.15 to 1.03, with detailed reduction, extraction, and analysis methods, including spectral evolution and redshift comparison.

Findings

Spectra show deeper intermediate mass element absorptions at z<0.5.

Redshift evolution is consistent with brighter, bluer supernovae at higher z.

Largest spectral sample in this redshift range to date.

Abstract

We present 139 spectra of 124 Type Ia supernovae (SNeIa) that were observed at the ESO/VLT during the first three years of the Canada-France-Hawai Telescope (CFHT) Supernova Legacy Survey (SNLS). This homogeneous data set is used to test for redshift evolution of SNeIa spectra, and will be used in the SNLS 3rd year cosmological analyses. Spectra have been reduced and extracted with a dedicated pipeline that uses photometric information from deep CFHT Legacy Survey (CFHT-LS) reference images to trace, at sub-pixel accuracy, the position of the supernova on the spectrogram as a function of wavelength. It also separates the supernova and its host light in 60% of cases. The identification of the supernova candidates is performed using a spectrophotometric SNIa model. A total of 124 SNeIa, roughly 50% of the overall SNLS spectroscopic sample, have been identified using the ESO/VLT during the first three years of the survey. Their redshifts range from z=0.149 to z=1.031. The average redshift of the sample is z=0.63+/-0.02. This constitutes the largest SNIa spectral set to date in this redshift range. The spectra are presented along with their best-fit spectral SNIa model and a host model where relevant. In the latter case, a host subtracted spectrum is also presented. We produce average spectra for pre-maximum, maximum and post-maximum epochs for both z<0.5 and z>=0.5 SNeIa. We find that z<0.5 spectra have deeper intermediate mass element absorptions than z>= 0.5 spectra. The differences with redshift are consistent with the selection of brighter and bluer supernovae at higher redshift.