HUBBLE RESIDUALS OF NEARBY TYPE Ia SUPERNOVAE ARE CORRELATED WITH HOST GALAXY MASSES

From Sloan Digital Sky Survey u'g'r'i'z' imaging, we estimate the stellar masses of the host galaxies of 70 low-redshift Type Ia supernovae (SNe Ia, 0.015 <z< 0.08) from the hosts' absolute luminosities and mass-to-light ratios. These nearby SNe were discovered largely by searches targeting luminous galaxies, and we find that their host galaxies are substantially more massive than the hosts of SNe discovered by the flux-limited Supernova Legacy Survey (SNLS). Testing four separate light curve fitters, we detect ∼2.5σ correlations of Hubble residuals with both host galaxy size and stellar mass, such that SNe Ia occurring in physically larger, more massive hosts are ∼10% brighter after light curve correction. The Hubble residual is the deviation of the inferred distance modulus to the SN, calculated from its apparent luminosity and light curve properties, away from the expected value at the SN redshift. Marginalizing over linear trends in Hubble residuals with light curve parameters shows that the correlations cannot be attributed to a light-curve-dependent calibration error. Combining 180 higher-redshift ESSENCE, SNLS, and HigherZ SNe with 30 nearby SNe whose host masses are less than 1010.8 M sun in a cosmology fit yields 1 + w = 0.22+0.152-0.108, while a combination where the 30 nearby SNe instead have host masses greater than 1010.8 M sun yields 1 + w = -0.03+0.217-0.143. Progenitor metallicity, stellar population age, and dust extinction correlate with galaxy mass and may be responsible for these systematic effects. Host galaxy measurements will yield improved distances to SNe Ia.