Paper detail

Cosmological parameters from the comparison of peculiar velocities with predictions from the 2M++ density field

Peculiar velocity measurements are the only tool available in the low-redshift Universe for mapping the large-scale distribution of matter and can thus be used to constrain cosmology. Using redshifts from the 2M++ redshift compilation, we reconstruct the density of galaxies within 200 Mpc/h, allowing for the first time good sampling of important superclusters such as the Shapley Concentration. We compare the predicted peculiar velocities from 2M++ to Tully-Fisher and SNe peculiar velocities. We find a value of $β^* \equiv Ω_{\rm{m}}^{0.55}/b^* = 0.431 \pm 0.021$, suggesting $Ω_{\rm{m}}^{0.55}σ_{\rm{8,lin}} = 0.401 \pm 0.024$, in good agreement with other probes. The predicted peculiar velocity of the Local Group arising from the 2M++ volume alone is $540 \pm 40$ km/s, towards $l = 268 \pm 4, b= 38 \pm 6$, only $10^\circ$ out of alignment with the Cosmic Microwave Background dipole. To account for velocity contributions arising from sources outside the 2M++ volume, we fit simultaneously for $β^*$ and an external bulk flow in our analysis. We find that an external bulk flow is preferred at the 5.1$σ$ level, and the best fit has a velocity of $159\pm23$ km/s towards $l=304 \pm 11, b = 6 \pm 13$. Finally, the predicted bulk flow of a 50 Mpc/h Gaussian-weighted volume centred on the Local Group is $230 \pm 30$ km/s, in the direction $l=293\pm 8, b = 14 \pm 10$, in agreement with predictions from $Λ$CDM.