Time Dilation in Type Ia Supernova Spectra at High Redshift

We present multiepoch spectra of 13 high-redshift Type Ia supernovae (SNe Ia) drawn from the literature, the ESSENCE and SNLS projects, and our own separate dedicated program on the ESO Very Large Telescope. We use the Supernova Identification (SNID) code of Blondin and Tonry to determine the spectral ages in the supernova rest frame. Comparison with the observed elapsed time yields an apparent aging rate consistent with the 1/(1 + z) factor (where z is the redshift) expected in a homogeneous, isotropic, expanding universe. These measurements thus confirm the expansion hypothesis, while unambiguously excluding models that predict no time dilation, such as Zwicky's "tired light" hypothesis. We also test for power-law dependencies of the aging rate on redshift. The best-fit exponent for these models is consistent with the expected 1/(1 + z) factor.

Based on observations made with ESO Telescopes at the Paranal Observatory under programs 67.A-0361, 267.A-5688, 078.D-0383, and 080.D-0477; at the Gemini Observatory and NOAO, which are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreements with the NSF; with the Magellan Telescopes at Las Campanas Observatory; and at the W. M. Keck Observatory, which was made possible by the generous financial support of the W. M. Keck Foundation.