Verifying the cosmological utility of Type Ia Supernovae : implications of a dispersion in the ultraviolet spectra

Creators: Ellis, R. S.; Sullivan, M.; Nugent, P. E.; Howell, D. A.; Gal-Yam, A.; Astier, P.; Balam, D.; Balland, C.; Basa, S.; Carlberg, R. G.; Conley, A.; Fouchez, D.; Guy, J.; Hardin, D.; Hook, I.; Pain, R.; Perrett, K.; Pritchet, C. J.; Regnault, N.

Abstract

We analyze the mean rest-frame ultraviolet (UV) spectrum of Type Ia Supernovae (SNe) and its dispersion using high signal-to-noise ratio Keck-I/LRIS-B spectroscopy for a sample of 36 events at intermediate redshift (ż = 0.5) discovered by the Canada-France-Hawaii Telescope Supernova Legacy Survey (SNLS). We introduce a new method for removing host galaxy contamination in our spectra, exploiting the comprehensive photometric coverage of the SNLS SNe and their host galaxies, thereby providing the first quantitative view of the UV spectral properties of a large sample of distant SNe Ia. Although the mean SN Ia spectrum has not evolved significantly over the past 40% of cosmic history, precise evolutionary constraints are limited by the absence of a comparable sample of high-quality local spectra. The mean UV spectrum of our z≃0.5 SNe Ia and its dispersion is tabulated for use in future applications. Within the high-redshift sample, we discover significant UV spectral variations and exclude dust extinction as the primary cause by examining trends with the optical SN color. Although progenitor metallicity may drive some of these trends, the variations we see are much larger than predicted in recent models and do not follow expected patterns. An interesting new result is a variation seen in the wavelength of selected UV features with phase. We also demonstrate systematic differences in the SN Ia spectral features with SN light curve width in both the UV and the optical. We show that these intrinsic variations could represent a statistical limitation in the future use of high-redshift SNe Ia for precision cosmology. We conclude that further detailed studies are needed, both locally and at moderate redshift where the rest-frame UV can be studied precisely, in order that future missions can confidently be planned to fully exploit SNe Ia as cosmological probes.

Additional Information

© 2008. The American Astronomical Society. Received 2007 September 5, accepted for publication 2007 October 19. The spectroscopic data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of theW.M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community.We are most fortunate to have the opportunity to conduct observations fromthis mountain. Additional observations were obtainedwithMegaPrime/MegaCam, a joint project of CFHTand CEA/DAPNIA, at the Canada-France- Hawaii Telescope, which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. Canadian collaborationmembers acknowledge support fromNSERC andCIAR; French collaboration members from CNRS/IN2P3, CNRS/INSU and CEA. P. E. N. acknowledges support from the US Department of Energy ScientificDiscovery throughAdvancedComputing program under contract DE-FG02-06ER06-04. A. G. acknowledges support by NASA through Hubble Fellowship grant HST-HF- 01158.01-A awarded by STScI, which is operated by AURA, Inc., for NASA, under contract NAS 5-26555. R. S. E. acknowledges financial support from DOE under contract DE-FG02- 04ER41316. This research used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the US Department of Energy under contract DE-AC03-76SF00098. We thank them for a generous allocation of computing time. We acknowledge useful discussions with Adam Riess, Mario Livio, and Ken Nomoto.

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