We present a measurement of the distant Type Ia supernova rate derived from the first two years of the Canada – France – Hawaii Telescope Supernova Legacy Survey .
We observed four one-square degree fields with a typical temporal frequency of \langle \Delta t \rangle \sim 4 observer-frame days over time spans of from 158 to 211 days per season for each field , with breaks during full moon .
We used 8-10 meter-class telescopes for spectroscopic followup to confirm our candidates and determine their redshifts .
Our starting sample consists of 73 spectroscopically verified Type Ia supernovae in the redshift range 0.2 < z < 0.6 .
We derive a volumetric SN Ia rate of r _ { V } ( \langle z \rangle = 0.47 ) = 0.42 ^ { +0.13 } _ { -0.09 } ( systematic ) \pm 0.06 ( statistical ) \times 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 } ( \langle z \rangle = 0.47 ) = 0.154 ^ { +0.048 } _ { -0.033 } ( systematic ) ^ { +0.039 } _ { -0.031 } ( statistical ) SNu .
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 _ { \odot } of stars formed .
Our rate and other rates from surveys using spectroscopic sample confirmation display only a modest evolution out to z = 0.55 .