Cosmological parameters from the comparison of peculiar velocities with predictions from the 2M++ density field
Résumé
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 h-1 Mpc, 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* ≡ Ω m^{0.55}/b^* = 0.431 ± 0.021, suggesting Ω m^{0.55}σ _{8,lin} = 0.401 ± 0.024, in good agreement with other probes. The predicted peculiar velocity of the Local Group arising from the 2M++ volume alone is 540 ± 40 km s-1, towards l = 268° ± 4°, b = 38° ± 6°, only 10° 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 5.1σ level, and the best fit has a velocity of 159 ± 23 km s- 1 towards l = 304° ± 11°, b = 6° ± 13°. Finally, the predicted bulk flow of a 50 h-1 Mpc Gaussian-weighted volume centred on the Local Group is 230 ± 30 km s-1, in the direction l = 293° ± 8°, b = 14° ± 10°, in agreement with predictions from Λ cold dark matter.
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