Constraining Cosmic Evolution of Type Ia Supernovae

We present the first large-scale effort of creating composite spectra of\nhigh-redshift type Ia supernovae (SNe Ia) and comparing them to low-redshift\ncounterparts. Through the ESSENCE project, we have obtained 107 spectra of 88\nhigh-redshift SNe Ia with excellent light-curve information. In addition, we\nhave obtained 397 spectra of low-redshift SNe through a multiple-decade effort\nat Lick and Keck Observatories, and we have used 45 UV spectra obtained by\nHST/IUE. The low-redshift spectra act as a control sample when comparing to the\nESSENCE spectra. In all instances, the ESSENCE and Lick composite spectra\nappear very similar. The addition of galaxy light to the Lick composite spectra\nallows a nearly perfect match of the overall spectral-energy distribution with\nthe ESSENCE composite spectra, indicating that the high-redshift SNe are more\ncontaminated with host-galaxy light than their low-redshift counterparts. This\nis caused by observing objects at all redshifts with the same slit width, which\ncorresponds to different projected distances. After correcting for the\ngalaxy-light contamination, subtle differences in the spectra remain. We have\nestimated the systematic errors when using current spectral templates for\nK-corrections to be ~0.02 mag. The variance in the composite spectra give an\nestimate of the intrinsic variance in low-redshift maximum-light SN spectra of\n~3% in the optical and growing toward the UV. The difference between the\nmaximum light low and high-redshift spectra constrain SN evolution between our\nsamples to be < 10% in the rest-frame optical.\n