Improved dark energy constraints from 100 ∼ new CfA supernova type Ia light curves

We combine the CfA3 supernovae Type Ia (SNIa) sample with samples from the literature to calculate improved constraints on the dark energy equation of state parameter, w. The CfA3 sample is added to the Union set of Kowalski et al. to form the Constitution set and, combined with a BAO prior, produces 1 + w = 0.013^+0.066 _-0.068 (0.11 syst), consistent with the cosmological constant. The CfA3 addition makes the cosmologically useful sample of nearby SNIa between 2.6 and 2.9 times larger than before, reducing the statistical uncertainty to the point where systematics play the largest role. We use four light-curve fitters to test for systematic differences: SALT, SALT2, MLCS2k2 (R_V = 3.1), and MLCS2k2 (R_V = 1.7). SALT produces high-redshift Hubble residuals with systematic trends versus color and larger scatter than MLCS2k2. MLCS2k2 overestimates the intrinsic luminosity of SNIa with 0.7 < Δ < 1.2. MLCS2k2 with R_V = 3.1 overestimates host-galaxy extinction while R_V ≈ 1.7 does not. Our investigation is consistent with no Hubble bubble. We also find that, after light-curve correction, SNIa in Scd/Sd/Irr hosts are intrinsically fainter than those in E/S0 hosts by 2σ, suggesting that they may come from different populations. We also find that SNIa in Scd/Sd/Irr hosts have low scatter (0.1 mag) and reddening. Current systematic errors can be reduced by improving SNIa photometric accuracy, by including the CfA3 sample to retrain light-curve fitters, by combining optical SNIa photometry with near-infrared photometry to understand host-galaxy extinction, and by determining if different environments give rise to different intrinsic SNIa luminosity after correction for light-curve shape and color.