A Measurement of the Rate of Type Ia Supernovae at Redshift z ≈ 0.1 from the First Season of the SDSS-II Supernova Survey

We present a measurement of the rate of Type Ia supernovae (SNe Ia) from the first of three seasons of data from the SDSS-II Supernova Survey. For this measurement, we include 17 SNe Ia at redshift z <= 0.12. Assuming a flat cosmology with Ω_m = 0.3 = 1 - Ω_Λ, we find a volumetric SN Ia rate of [ 2.93^{+ 0.17}_-0.04(systematic)^{+ 0.90}_-0.71(statistical) ] × 10^-5 SNe Mpc ^-3 h³₇₀ yr ^-1, at a volume-weighted mean redshift of 0.09. This result is consistent with previous measurements of the SN Ia rate in a similar redshift range. The systematic errors are well controlled, resulting in the most precise measurement of the SN Ia rate in this redshift range. We use a maximum likelihood method to fit SN rate models to the SDSS-II Supernova Survey data in combination with other rate measurements, thereby constraining models for the redshift evolution of the SN Ia rate. Fitting the combined data to a simple power-law evolution of the volumetric SN Ia rate, r_V propto (1 + z)^β, we obtain a value of β = 1.5 +/- 0.6, i.e., the SN Ia rate is determined to be an increasing function of redshift at the ~2.5 σ level. Fitting the results to a model in which the volumetric SN rate is r_V = Aρ(t) + Bdot rho(t), where ρ (t) is the stellar mass density and dot rho(t) is the star formation rate, we find A = (2.8 +/- 1.2) × 10^-14 SNe M^-1_⊙ yr ^-1, B = (9.3^{+ 3.4}_-3.1) × 10^-4 SNe M^-1_⊙.