THE SDSS-II SUPERNOVA SURVEY: PARAMETERIZING THE TYPE Ia SUPERNOVA RATE AS A FUNCTION OF HOST GALAXY PROPERTIES

Using data from the Sloan Digital Sky Supernova Survey-II (SDSS-II SN Survey), we measure the rate of Type Ia supernovae (SNe Ia) as a function of galaxy properties at intermediate redshift. A sample of 342 SNe Ia with 0.05 < z < 0.25 is constructed. Using broadband photometry and redshifts, we use the P GASE.2 spectral energy distributions to estimate host galaxy stellar masses and recent star formation rates (SFRs). We find that the rate of SNe Ia per unit stellar mass is significantly higher (by a factor of 30) in highly star-forming galaxies compared to passive galaxies. When parameterizing the SN Ia rate (SNR Ia ) based on host galaxy properties, we find that the rate of SNe Ia in passive galaxies is not linearly proportional to the stellar mass; instead an SNR Ia M 0.68 is favored. However, such a parameterization does not describe the observed SNR Ia in star-forming galaxies. The SNR Ia in star-forming galaxies is well fitted by SNR Ia = (0.41 0.15) 10 -10 M 0.720.15 + (0.65 0.25) 10 -3 SFR 1.010.22 (statistical errors only), where M is the host galaxy stellar mass (in M ) and SFR is the SFR (in M yr -1 ). We show that our results, for SNe Ia in passive galaxies, are consistent with those at higher redshifts (favoring SNR Ia M) when accounting for the difference in the ages of our galaxies. This suggests that the rate of SNe Ia is correlated with the age of the stellar population. The MLCS extinction parameter, A V , is similar in passive and moderately star-forming galaxies, but we find indications that it is smaller, on average, in highly star-forming galaxies. This result appears to be driven by a deficit of the reddest (A V > 0.15) SNe Ia in highly star-forming galaxies. We consider that the high levels of dust in these systems may be obscuring the reddest and faintest SNe Ia.