SNLS3: Constraints on Dark Energy Combining the Supernova Legacy Survey
  Three Year Data with Other Probes
  We present observational constraints on the nature of dark energy using the
Supernova Legacy Survey three year sample (SNLS3) of Guy et al. (2010) and
Conley et al. (2011). We use the 472 SNe Ia in this sample, accounting for
recently discovered correlations between SN Ia luminosity and host galaxy
properties, and include the effects of all identified systematic uncertainties
directly in the cosmological fits. Combining the SNLS3 data with the full WMAP7
power spectrum, the Sloan Digital Sky Survey luminous red galaxy power
spectrum, and a prior on the Hubble constant H0 from SHOES, in a flat universe
we find omega_m=0.269+/-0.015 and w=-1.061+0.069-0.068 -- a 6.5% measure of the
dark energy equation-of-state parameter w. The statistical and systematic
uncertainties are approximately equal, with the systematic uncertainties
dominated by the photometric calibration of the SN Ia fluxes -- without these
calibration effects, systematics contribute only a ~2% error in w. When
relaxing the assumption of flatness, we find omega_m=0.271+/-0.015,
omega_k=-0.002+/-0.006, and w=-1.069+0.091-0.092. Parameterizing the time
evolution of w as w(a)=w_0+w_a(1-a), gives w_0=-0.905+/-0.196,
w_a=-0.984+1.094-1.097 in a flat universe. All of our results are consistent
with a flat, w=-1 universe. The size of the SNLS3 sample allows various tests
to be performed with the SNe segregated according to their light curve and host
galaxy properties. We find that the cosmological constraints derived from these
different sub-samples are consistent. There is evidence that the coefficient,
beta, relating SN Ia luminosity and color, varies with host parameters at
>4sigma significance (in addition to the known SN luminosity--host relation);
however this has only a small effect on the cosmological results and is
currently a sub-dominant systematic.
