The Distant Type Ia Supernova Rate

We present a measurement of the rate of distant Type Ia supernovae derivedusing 4 large subsets of data from the Supernova Cosmology Project. Within thisfiducial sample, which surveyed about 12 square degrees, thirty-eightsupernovae were detected at redshifts 0.25--0.85. In a spatially-flatcosmological model consistent with the results obtained by the SupernovaCosmology Project, we derive a rest-frame Type Ia supernova rate at a meanredshift $z\simeq0.55$ of $1.53 {^{+0.28}_{-0.25}} {^{+0.32}_{-0.31}} 10^{-4}h^3 {\rm Mpc}^{-3} {\rm yr}^{-1}$ or $0.58 {^{+0.10}_{-0.09}}{^{+0.10}_{-0.09}} h^2 {\rm SNu}$ (1 SNu = 1 supernova per century per$10^{10}$\Lbsun), where the first uncertainty is statistical and the secondincludes systematic effects. The dependence of the rate on the assumedcosmological parameters is studied and the redshift dependence of the rate perunit comoving volume is contrasted with local estimates in the context ofpossible cosmic star formation histories and progenitor models.