The ESO/VLT 3rd year Type Ia supernova data set from the supernova legacy survey

Aims: We present 139 spectra of 124 Type Ia supernovae (SNe Ia) 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 SN Ia spectra, and will be used in the SNLS 3rd year cosmological analyses. Methods: 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 SN Ia model. Results: A total of 124 SNe Ia, 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 SN Ia spectral set to date in this redshift range. The spectra are presented along with their best-fit spectral SN Ia 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 SNe Ia. 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. Based on observations obtained with FORS1 and FORS2 at the Very Large Telescope on Cerro Paranal, operated by the European Southern Observatory, Chile (ESO Large Programs 171.A-0486 and 176.A-0589). Appendix is only available in electronic form at http://www.aanda.org