Lawrence Berkeley National Laboratory

We analyze the mean rest-frame ultraviolet (UV) spectrum of Type Ia Supernovae(SNe) and its dispersion using high signal-to-noise Keck-I/LRIS-B spectroscopyfor a sample of 36 events at intermediate redshift (z=0.5) discoveredby the Canada-France-Hawaii Telescope Supernova Legacy Survey (SNLS). Weintroduce a new method for removing host galaxy contamination in our spectra,exploiting the comprehensive photometric coverage of the SNLS SNe and theirhost galaxies, thereby providing the first quantitative view of the UV spectralproperties of a large sample of distant SNe Ia. Although the mean SN Ia spectrumhas not evolved significantly over the past 40 percent of cosmic history, preciseevolutionary constraints are limited by the absence of a comparable sample ofhigh quality local spectra. The mean UV spectrum of our z 0.5 SNe Ia and itsdispersion is tabulated for use in future applications. Within the high-redshiftsample, we discover significant UV spectral variations and exclude dust extinctionas the primary cause by examining trends with the optical SN color. Although progenitor metallicity may drive some of these trends, the variations we see aremuch 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 UVfeatures with phase. We also demonstrate systematic differences in the SN Iaspectral features with SN lightcurve width in both the UV and the optical. Weshow that these intrinsic variations could represent a statistical limitation in thefuture use of high-redshift SNe Ia for precision cosmology. We conclude thatfurther detailed studies are needed, both locally and at moderate redshift wherethe rest-frame UV can be studied precisely, in order that future missions canconfidently be planned to fully exploit SNe Ia as cosmological probes.