MEASUREMENTS OF THE RATE OF TYPE Ia SUPERNOVAE AT REDSHIFT ≲0.3 FROM THE SLOAN DIGITAL SKY SURVEY II SUPERNOVA SURVEY

We present a measurement of the volumetric Type Ia supernova (SN Ia) rate\nbased on data from the Sloan Digital Sky Survey II (SDSS-II) Supernova Survey.\nThe adopted sample of supernovae (SNe) includes 516 SNe Ia at redshift z\n\\lesssim 0.3, of which 270 (52%) are spectroscopically identified as SNe Ia.\nThe remaining 246 SNe Ia were identified through their light curves; 113 of\nthese objects have spectroscopic redshifts from spectra of their host galaxy,\nand 133 have photometric redshifts estimated from the SN light curves. Based on\nconsideration of 87 spectroscopically confirmed non-Ia SNe discovered by the\nSDSS-II SN Survey, we estimate that 2.04+1.61-0.95 % of the photometric SNe Ia\nmay be misidentified. The sample of SNe Ia used in this measurement represents\nan order of magnitude increase in the statistics for SN Ia rate measurements in\nthe redshift range covered by the SDSS-II Supernova Survey. If we assume a SN\nIa rate that is constant at low redshift (z < 0.15), then the SN observations\ncan be used to infer a value of the SN rate of rV = (2.69+0.34+0.21-0.30-0.01)\nx10^{-5} SNe yr^{-1} Mpc-3 (H0 /(70 km s^{-1} Mpc^{-1}))^{3} at a mean redshift\nof ~ 0.12, based on 79 SNe Ia of which 72 are spectroscopically confirmed.\nHowever, the large sample of SNe Ia included in this study allows us to place\nconstraints on the redshift dependence of the SN Ia rate based on the SDSS-II\nSupernova Survey data alone. Fitting a power-law model of the SN rate\nevolution, r_V(z) = A_p x ((1 + z)/(1 + z0))^{\\nu}, over the redshift range 0.0\n< z < 0.3 with z0 = 0.21, results in A_p = (3.43+0.15-0.15) x 10^{-5} SNe\nyr^{-1} Mpc-3 (H0 /(70 km s^{-1} Mpc^{-1}))^{3} and \\nu = 2.04+0.90-0.89.\n