The Type Ia Supernova Rate atz ≈ 0.5 from the Supernova Legacy Survey

We present a measurement of the distant Type Ia supernova (SN Ia) rate derived from the first 2 yr of the Canada-France-Hawaii Telescope Supernova Legacy Survey. We observed four 1° × 1° fields with a typical temporal frequency of ⟨Δ t ⟩ ∼ 4 observer-frame days over time spans of 158-211 days per season for each field, with breaks during the full Moon. We used 8-10 m class telescopes for spectroscopic follow-up to confirm our candidates and determine their redshifts. Our starting sample consists of 73 spectroscopically verified SNe Ia in the redshift range 0.2 < z < 0.6. We derive a volumetric SN Ia rate of r V (⟨ z ⟩ = 0.47) = × 10 -4 yr -1 Mpc 3 , assuming h = 0.7, Ω m = 0.3, and a flat cosmology. Using recently published galaxy luminosity functions derived in our redshift range, we derive a SN Ia rate per unit luminosity of r L (⟨ z ⟩ = 0.47) = 0.154 (syst.) (stat.) SN units. Using our rate alone, we place an upper limit on the component of SN Ia production that tracks the cosmic star formation history of 1 SN Ia per 10 3 M ⊙ of stars formed. Our rate and other rates from surveys using spectroscopic sample confirmation display only a modest evolution out to z = 0.55.