SNLS spectroscopy: testing for evolution in type Ia supernovae

Abstract

Aims. We present a quantitative study of a new data set of high redshift Type Ia supernovae spectra, observed at the Gemini telescopes during the first 34 months of the Supernova Legacy Survey. During this time 123 supernovae candidates were observed, of which 87 have been identified as SNe Ia at a median redshift of z = 0.720. Spectra from the entire second year of the survey and part of the third year (59 total SNe candidates with 46 confirmed SNe Ia) are published here for the first time. The spectroscopic measurements made on this data set are used determine if these distant SNe comprise a population similar to those observed locally. Methods. Rest-frame equivalent width and ejection velocity measurements are made on four spectroscopic features. Corresponding measurements are presented for a set of 167 spectra from 24 low-z SNe Ia from the literature. Results. We show that there exists a sample at high redshift with properties similar to nearby SNe. The high-z measurements are consistent with the range of measurements at low-z and no significant difference was found between the distributions of measurements at low and high redsift for three of the features. The fourth feature displays a possible difference that should be investigated further. Correlations between Type Ia SNe properties and host galaxy morphology were also found to be similar at low and high z, and within each host galaxy class we see no evidence for redshift-evolution in SN properties. A new correlation between SNe Ia peak magnitude and the equivalent width of SiII absorption is presented. Tests on a sub-set of the SNLS SNe demonstrates that this correlation reduces the scatter in SNe Ia luminosity distances in a manner consistent with the lightcurve shape-luminosity corrections that are used for Type Ia SNe cosmology. Conclusions. We show that this new sample of SNLS SNe Ia has spectroscopic properties similar to nearby objects.

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

“…However, the presence of Si ii λ4000 in the Gemini spectrum contradicts this hypothesis, as this feature is not seen in under-luminous SNe because of extra absorption due to Ti ii and Fe ii in the same wavelength range. Based on this observation, Bronder et al (2008) concluded that 03D1co was likely to be a "normal" SN Ia, in agreement with its normal light curves (Astier et al 2006). The SALT2 fit of the VLT spectrum of 03D1co confirms this conclusion (see Fig.…”

“…While the SN Ia category represents two thirds of our total sample, the number of SNe identified as "peculiar" is very small, which may reflect selection biases against such events (Bronder et al 2008). Low-stretch, underluminous SNe Ia are very hard to identify spectroscopically because of their dimness compared to their usually bright host.…”

“…Almost 50% of SNLS candidates identified as certain or probable SN Ia were spectroscopically observed at the VLT. Howell et al (2005) and Bronder et al (2008) describe the SNLS first three years of Gemini spectral data (up to May 2006), while Ellis et al (2008) present 36 high signal-to-noise ratio (S /N) SN Ia spectra obtained at Keck. In this paper, we focus on spectra taken at the VLT on Cerro Paranal.…”

“…Bronder et al (2008) used deep i M -band photometry to estimate host contamination. More recently, Ellis et al (2008) used the multi-colour SNLS photometry to do a more sophisticated estimate of host contamination in extracting Keck SNe Ia spectra.…”

“…In case of disagreement, the most conservative typing was chosen. This procedure makes the identification of the SNe Ia presented in this paper homogeneous to the identification of Gemini spectra of Howell et al (2005) and Bronder et al (2008). Note however that we do not use here the confidence index (CI) classification of Howell et al (2005).…”