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We present Keck high-quality rest-frame ultraviolet (UV) through opticalspectra of 21 Type Ia supernovae (SNe Ia) in the redshift range 0.11 < z < 0.37and a mean redshift of 0.22 that were discovered during the Sloan Digital SkySurvey-II (SDSS-II) SN Survey. Using the broad-band photometry of the SDSSsurvey, we are able to reconstruct the SN host-galaxy spectral energydistributions (SEDs), allowing for a correction for the host-galaxycontamination in the SN Ia spectra. Comparison of composite spectra constructedfrom a subsample of 17 high-quality spectra to those created from alow-redshift sample with otherwise similar properties shows that the Keck/SDSSSNe Ia have, on average, extremely similar rest-frame optical spectra but showa UV flux excess. This observation is confirmed by comparing synthesizedbroad-band colors of the individual spectra, showing a difference in meancolors at the 2.4 - 4.4 sigma level for various UV colors. We further see aslight difference in the UV spectral shape between SNe with low-mass andhigh-mass host galaxies. Additionally, we detect a relationship between theflux ratio at 2770 and 2900 A and peak luminosity that differs from thatobserved at low redshift. We find that changing the UV SED of an SN Ia withinthe observed dispersion can change the inferred distance moduli by ~0.1 mag.This effect only occurs when the data probe the rest-frame UV. We suggest thatthis discrepancy could be due to differences in the host-galaxy population ofthe two SN samples or to small-sample statistics.

the astronomical journal

A Mismatch in the Ultraviolet Spectra between Low-Redshift and  Intermediate-Redshift Type Ia Supernovae as a Possible Systematic Uncertainty  for Supernova Cosmology