Spectral Properties of High-Redshift Type Ia Supernovae - Expansion Velocities

We present the first results of an analysis carried out on 53 high-redshift Type Ia supernova (SN Ia) spectra taken by members of the ESSENCE collaboration and presented in Matheson et al. (2005). We measure expansion velocities associated with spectroscopic features characteristic of SNe Ia (Ca II λ 3945, S II λ 5640, and Si II λ 6355) and find their time-evolution to be entirely consistent with those of SNe Ia in the local Universe (z ≲ 0.01). We show that the expansion velocities of the weak S II λ 5640 line can in principle be used to discriminate between sub-luminous, normal, and over-luminous SNe Ia. Though this result holds for nearby SNe Ia, the current errors associated with such measurements in their high-z counterparts preclude the use of this diagnostic tool to investigate systematic differences between SNe Ia at different redshifts. We also give the first direct evidence for the presence of detached high-velocity Ca II in a high-z SN Ia spectrum.

This work is partially supported by grant AST-0443378 from the US National Science Foundation.