The type Ia supernova rate at z≈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 degrees x 1 degrees fields with a typical temporal frequency of <Delta t > similar to 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) = [0.42(-0.09)(+0.13)(syst.) +/- 0.06(stat.)] x 10(-4) yr(-1) Mpc(3), assuming h = 0: 7, Omega(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(-0.033)(+0.048)(syst:)(-0.031)(+0.039)(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 circle dot of stars formed. Our rate and other rates from surveys using spectroscopic sample confirmation display only a modest evolution out to z = 0.55.