NTT and NOT spectroscopy of SDSS-II supernovae

¹ Department of PhysicsStockholm University, 106 91 Stockholm, Sweden
e-mail: linda@ifae.es
² Institut de Física d’Altes Energies, 08193 Bellaterra, Barcelona, Spain
³ Oskar Klein Centre for Cosmo Particle Physics, AlbaNova University Center, 106 91 Stockholm, Sweden
⁴ Institute of Cosmology and Gravitation, Portsmouth PO13 FX, UK
⁵ Department of Mathematics and Applied Mathematics, University of Cape Town, South Africa
⁶ Astronomy Department, Stockholm University, 106 91 Stockholm, Sweden
⁷ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen Ø, Denmark
⁸ CENTRA – Centro Multidisciplinar de Astrofísica, Instituto Superior Técnico, 1049-001 Lisbon, Portugal
⁹ Carnegie Institute for Science, Carnegie Observatories, Casilla 601, La Serena, Chile
¹⁰ South African Astronomical Observatory, Cape Town, South Africa
¹¹ African Institute for Mathematical Sciences, Muizenberg, Cape Town, South Africa
¹² School of Mathematics and Physics, University of Queensland, QLD 4072, Australia
¹³ Center for Particle Astrophysics, Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
¹⁴ Kavli Institute for Cosmological Physics, University of Chicago, Chicago, Illinois 60637, USA
¹⁵ University of Notre Dame, Notre Dame, IN 46556-5670, USA
¹⁶ Department of Physics and Astronomy, University of Sussex, UK
¹⁷ Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104, USA
¹⁸ Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802 USA
¹⁹ Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA 94305-4060, USA

Received: 7 September 2010
Accepted: 16 October 2010

Abstract

Context. The Sloan Digital Sky Survey II (SDSS-II) Supernova Survey, conducted between 2005 and 2007, was designed to detect a large number of type Ia supernovae around z ~ 0.2, the redshift “gap” between low-z and high-z supernova searches. The survey has provided multi-band (ugriz) photometric lightcurves for variable targets, and supernova candidates were scheduled for spectroscopic observations, primarily to provide supernova classification and accurate redshifts. We present supernova spectra obtained in 2006 and 2007 using the New Technology Telescope (NTT) and the Nordic Optical Telescope (NOT).

Aims. We provide an atlas of supernova spectra in the range z  =  0.03–0.32 that complements the well-sampled lightcurves from SDSS-II in the forthcoming three-year SDSS supernova cosmology analysis. The sample can, for example, be used for spectral studies of type Ia supernovae, which are critical for understanding potential systematic effects when supernovae are used to determine cosmological distances.

Methods. The spectra were reduced in a uniform manner, and special care was taken in estimating the uncertainties for the different processing steps. Host-galaxy light was subtracted when possible and the supernova type fitted using the SuperNova IDentification code (SNID). We also present comparisons between spectral and photometric dating using SALT lightcurve fits to the photometry from SDSS-II, as well as the global distribution of our sample in terms of the lightcurve parameters: stretch and colour.

Results. We report new spectroscopic data from 141 type Ia supernovae, mainly between –9 and  +15 days from lightcurve maximum, including a few cases of multi-epoch observations. This homogeneous, host-galaxy subtracted, type Ia supernova spectroscopic sample is among the largest such data sets and unique in its redshift interval. The sample includes two potential SN 1991T-like supernovae (SN 2006on and SN 2007ni) and one potential SN 2002cx-like supernova (SN 2007ie). In addition, the new compilation includes spectra from 23 confirmed type II and 8 type Ib/c supernovae.