Spectral properties of type Ia supernovae up to z  ~  0.3^⋆,⋆⋆

¹ Department of PhysicsStockholm University, 10691 Stockholm, Sweden
e-mail: nordin@physto.se
² Oskar Klein Centre for Cosmo Particle Physics, AlbaNova, 10691 Stockholm, Sweden
³ Institut de Física d’Altes Energies, 08193 Bellaterra, Barcelona, Spain
⁴ Institute of Cosmology and Gravitation, Portsmouth PO13 FX, UK
⁵ Department of Mathematics and Applied Mathematics, University of Cape Town, Cape Town, South Africa
⁶ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark
⁷ Astronomy Department, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
⁸ South African Astronomical Observatory, Cape Town, South Africa
⁹ African Institute for Mathematical Sciences, Muizenberg, Cape Town, South Africa
¹⁰ Center for Particle Astrophysics, Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
¹¹ Kavli Institute for Cosmological Physics, University of Chicago, Chicago, Illinois 60637, USA
¹² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹³ Department of Physics, 225 Nieuwland Science Hall, University of Notre Dame, Notre Dame, IN 46556, USA
¹⁴ Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104, USA
¹⁵ Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802 USA

Received: 7 September 2010
Accepted: 18 November 2010

Abstract

Aims. Spectroscopic observations of type Ia supernovae obtained at the New Technology Telescope (NTT) and the Nordic Optical Telescope (NOT), in conjunction with the SDSS-II Supernova Survey, are analysed. We use spectral indicators measured up to a month after the lightcurve peak luminosity to characterise the supernova properties, and examine these for potential correlations with host galaxy type, lightcurve shape, colour excess, and redshift.

Methods. Our analysis is based on 89 type Ia supernovae at a redshift interval z = 0.05−0.3, for which multiband SDSS photometry is available. A lower-z spectroscopy reference sample was used for comparisons over cosmic time. We present measurements of time series of pseudo equivalent widths and line velocities of the main spectral features in type Ia supernovae.

Results. Supernovae with shallower features are found predominantly among the intrinsically brighter slow declining supernovae. We detect the strongest correlation between lightcurve stretch and the Si ii λ4000 absorption feature, which also correlates with the estimated mass and star formation rate of the host galaxy. We also report a tentative correlation between colour excess and spectral properties. If confirmed, this would suggest that moderate reddening of type Ia supernovae is dominated by effects in the explosion or its immediate environment, as opposed to extinction by interstellar dust.