The ESO’s VLT type Ia supernova spectral set of the final two years of SNLS^{★,★★,★★★}

¹ Sorbonne Université, Laboratoire de Physique Nucléaire et de Hautes Energies, CNRS-IN2P3, 4 Place Jussieu, 75005 Paris, France
e-mail: balland@lpnhe.in2p3.fr
² Australian Astronomical Observatory, Epping, New South Wales, Australia
³ ARC Centre of Excellence for All-sky Astrophysics (CAASTRO), Canberra, Australia
⁴ Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada
⁵ Center for Astrophysics and Space Astronomy 389-UCB, University of Colorado, Boulder, CO 80309, USA
⁶ European Southern Observatory (ESO), Karl-Schwarzschild Strasse 2, 85748 Garching, Germany
⁷ Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK
⁸ Department of Physics (Astrophysics), University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁹ Department of Physics, Lancaster University, Lancaster LA1 4YB, UK
¹⁰ Las Cumbres Observatory, 6740 Cortona Dr., Suite 102, Goleta, CA 93117, USA
¹¹ Department of Physics, University of California, Santa Barbara, Broida Hall, Mail Code 9530, Santa Barbara, CA 93106-9530, USA
¹² NRC Herzberg Institute for Astrophysics, 5071 West Saanich Road, Victoria V9E 2E7, British Columbia, Canada
¹³ Department of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1SX, UK
¹⁴ Seven Bridges Genomics Inc., 1 Main St, 5th Floor, Suite 500, Cambridge, MA 02142, USA
¹⁵ CENTRA – Centro Multidisciplinar de Astrofísica and Dep. Física, Instituto Superior Técnico, Universidade de Lisboa, Portugal
¹⁶ Aix Marseille Univ, CNRS/IN2P3, CPPM, Marseille, France
¹⁷ Research School of Astronomy and Astrophysics, Australian National University, Canberra, Australia
¹⁸ LAL, Univ. Paris-Sud, CNRS-IN2P3, Université Paris-Saclay, Orsay, France
¹⁹ CEA, Centre de Saclay, Irfu/SPP, 91191 Gif-sur-Yvette, France

Received: 10 September 2017
Accepted: 6 December 2017

Abstract

Aims. We aim to present 70 spectra of 68 new high-redshift type Ia supernovae (SNe Ia) measured at ESO’s VLT during the final two years of operation (2006–2008) of the Supernova Legacy Survey (SNLS). This new sample complements the VLT three year spectral set. Altogether, these two data sets form the five year sample of SNLS SN Ia spectra measured at the VLT on which the final SNLS cosmological analysis will partly be based. In the redshift range considered, this sample is unique in terms of homogeneity and number of spectra. We use it to investigate the possibility of a spectral evolution of SNe Ia populations with redshift as well as SNe Ia spectral properties as a function of lightcurve fit parameters and the mass of the host-galaxy.

Methods. Reduction and extraction are based on both IRAF standard tasks and our own reduction pipeline. Redshifts are estimated from host-galaxy lines whenever possible or alternatively from supernova features. We used the spectro-photometric SN Ia model SALT2 combined with a set of galaxy templates that model the host-galaxy contamination to assess the type Ia nature of the candidates.

Results. We identify 68 new SNe Ia with redshift ranging from z = 0.207 to z = 0.98 for an average redshift of z = 0.62. Each spectrum is presented individually along with its best-fit SALT2 model. Adding this new sample to the three year VLT sample of SNLS, the final dataset contains 209 spectra corresponding to 192 SNe Ia identified at the VLT. We also publish the redshifts of other candidates (host galaxies or other transients) whose spectra were obtained at the same time as the spectra of live SNe Ia. This list provides a new redshift catalog useful for upcoming galaxy surveys. Using the full VLT SNe Ia sample, we build composite spectra around maximum light with cuts in color, the lightcurve shape parameter (“stretch”), host-galaxy mass and redshift. We find that high-z SNe Ia are bluer, brighter and have weaker intermediate mass element absorption lines than their low-z counterparts at a level consistent with what is expected from selection effects. We also find a flux excess in the range [3000–3400] Å for SNe Ia in low mass host-galaxies (M < 10¹⁰M_⊙) or with locally blue U–V colors, and suggest that the UV flux (or local color) may be used in future cosmological studies as a third standardization parameter in addition to stretch and color.