The rise time of Type Ia supernovae from the Supernova Legacy Survey

We compare the rise times of nearby and distant Type Ia supernovae (SNe Ia) as a test for evolution using 73 high-redshift spectroscopically confirmed SNe Ia from the first 2 years of the 5 year Supernova Legacy Survey (SNLS) and published observations of nearby SNe. Because of the rolling search nature of the SNLS, our measurement is approximately 6 times more precise than previous studies, allowing for a more sensitive test of evolution between nearby and distant SNe. Adopting a simple t(2) early-time model (as in previous studies), we find that the rest-frame B rise times for a fiducial SN Ia at high and low redshift are consistent, with values 19.10(-0.17)(+0.18) (stat) +/- 0.2(syst) and 19.58(-0.19)(+0.22) days, respectively; the statistical significance of this difference is only 1.4 sigma. The errors represent the uncertainty in the mean rather than any variation between individual SNe. We also compare subsets of our high-redshift data set based on decline rate, host galaxy star formation rate, and redshift, finding no substantive evidence for any subsample dependence.