The Peculiar Velocities of Local Type Ia Supernovae and Their Impact on Cosmology

We quantify the effect of Type Ia supernova (SN Ia) peculiar velocities on the derivation of cosmological parameters. The published distant and local SNe Ia used for the Supernova Legacy Survey first-year cosmology report form the sample for this study. While previous work has assumed that the local SNe are at rest in the CMB frame (the no-flow assumption), we test this assumption by applying peculiar velocity corrections to the local SNe using three different flow models. The models are based on the IRAS PSCz galaxy redshift survey, have varying β = Ω^(0.6)_m/b, and reproduce the Local Group motion in the CMB frame. These data sets are then fit for w, Ω_m, and Ω_Λ using flatness or ΛCDM and a BAO prior, and the χ^2 statistic is used to examine the effect of the velocity corrections on the quality of the fits. The most favored model is the β = 0.5 model, which produces a fit significantly better than the no-flow assumption, consistent with previous peculiar velocity studies. By comparing the no-flow assumption with the favored models, we derive the largest potential systematic error in w caused by ignoring peculiar velocities, Δw = +0.04. For Ω_Λ the potential error is ΔΩ_Λ = -0.04, and for Ω_m the potential error is ΔΩ_m < +0.01. The favored flow model (β = 0.5) produces the following cosmological parameters: w = -1.08^(+0.09)_(-0.08), Ω_m = 0.27^(+0.02)_(-0.02) assuming a flat cosmology, and Ω_Λ = 0.80^(0.08)_(-0.07) and Ω_m = 0.27^(+0.02)_(-0.02) for a w = -1 (ΛCDM) cosmology.