Supernova Constraints and Systematic Uncertainties from the First 3
  Years of the Supernova Legacy Survey
  We combine high redshift Type Ia supernovae from the first 3 years of the
Supernova Legacy Survey (SNLS) with other supernova (SN) samples, primarily at
lower redshifts, to form a high-quality joint sample of 472 SNe (123 low-$z$,
93 SDSS, 242 SNLS, and 14 {\it Hubble Space Telescope}). SN data alone require
cosmic acceleration at >99.9% confidence, including systematic effects. For the
dark energy equation of state parameter (assumed constant out to at least
$z=1.4$) in a flat universe, we find $w = -0.91^{+0.16}_{-0.20}(\mathrm{stat})
^{+0.07}_{-0.14} (\mathrm{sys})$ from SNe only, consistent with a cosmological
constant. Our fits include a correction for the recently discovered
relationship between host-galaxy mass and SN absolute brightness. We pay
particular attention to systematic uncertainties, characterizing them using a
systematics covariance matrix that incorporates the redshift dependence of
these effects, as well as the shape-luminosity and color-luminosity
relationships. Unlike previous work, we include the effects of systematic terms
on the empirical light-curve models. The total systematic uncertainty is
dominated by calibration terms. We describe how the systematic uncertainties
can be reduced with soon to be available improved nearby and
intermediate-redshift samples, particularly those calibrated onto
USNO/SDSS-like systems.
