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

TL;DR

This study uses peculiar velocity data and the 2M++ galaxy density field to estimate cosmological parameters, find the Local Group's velocity, and analyze external bulk flows, confirming consistency with ΛCDM predictions.

Contribution

First, it reconstructs the galaxy density within 200 Mpc/h using 2M++ redshifts, enabling detailed analysis of superclusters; second, it compares predicted peculiar velocities with observations to constrain cosmological parameters; third, it assesses external bulk flows affecting local velocities.

Findings

Estimated 7^* = 0.431 b1 0.021, consistent with other probes.

Predicted Local Group velocity is 540 b1 40 km/s, aligned within 10b0 of the CMB dipole.

Detected a significant external bulk flow of 159 b1 23 km/s.

Abstract

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 $\beta^* \equiv \Omega_{\rm{m}}^{0.55}/b^* = 0.431 \pm 0.021$, suggesting $\Omega_{\rm{m}}^{0.55}\sigma_{\rm{8,lin}} = 0.401 \pm 0.024$, 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 $\beta^*$ and an external bulk flow in our analysis. We find that an external bulk flow is preferred at the 5.1$\sigma$ 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 $\Lambda$CDM.