Type Ia supernova luminosities

Selecting SNIa based on host photometry quality, we then performed cosmological fits using local color as a third standardization variable, for which we split the sample at the median value. We find a local color step significance of -0.091+/-0.013mag (7{sigma}), which effect is as significant as the maximum mass step effect. This indicates that the remaining luminosity variations in SNIa samples can be reduced with a third standardization variable that takes the environment into account.

Correcting for the maximum mass step correction of -0.094+/-0.013mag, we find a local color effect of -0.057+/-0.012mag (5{sigma}), which shows that additional information is provided by the close environment of SNIa. Departures from the initial choices were investigated and showed that the local color effect is still present, although less pronounced. We discuss the possible implications for cosmology and find that using the local color in place of the stellar mass results in a change in the measured value of the dark energy equation-of-state parameter of 0.6%.

Standardization using local U-V color in addition to stretch and color reduces the total dispersion in the Hubble diagram from 0.15 to 0.14mag. This will be of tremendous importance for the forthcoming SNIa surveys, and in particular for the Large Synoptic Survey Telescope (LSST), for which uncertainties on the dark energy equation of state will be comparable to the effects reported here.