Dependence of Type Ia supernova luminosities on their local environment
  We present a fully consistent catalog of local and global properties of host
galaxies of 882 Type Ia supernov\ae\ (SNIa) that were selected based on their
light-curve properties, spanning the redshift range $0.01 < z < 1.\text{}$ This
catalog corresponds to a preliminary version of the compilation sample and
includes Supernova Legacy Survey (SNLS) 5-year data, Sloan Digital Sky Survey
(SDSS), and low-redshift surveys. We measured low- and moderate-redshift host
galaxy photometry in SNLS and SDSS images and used spectral energy distribution
(SED) fitting techniques to derive host properties such as stellar mass and
$U-V$ rest-frame colors; the latter are an indicator of the luminosity-weighted
age of the stellar population in a galaxy. We also estimated the local observed
fluxes at the supernova location within a proper distance radius of 3 kpc, and
transposed them into local $U-V$ rest-frame colors. 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 as significant as the maximum mass
step effect. Correcting for the maximum mass step correction, we still find a
significant local color effect, which shows that additional information is
provided by the close environment of SNIa. Departures from the initial choices
were investigated, and we discuss the possible implications for cosmology. 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.
