Virtues of the Visible; Voluminous Vistas & Velocities

Abstract

Peculiar velocities are the motions of objects in excess of that resulting from the expansion of the Universe. They arise from gravitational interactions, and as such, probe the underlying distribution of dark matter on large scales. Early work in this field was in part driven to explain the peculiar motion of the Local Group of galaxies (LG) inferred from the dipole in the cosmic microwave background (CMB) to be 627 ± 22 km/s towards l = 276° ± 3°, b = 30° ± 2°. Recent work has yet to fully account for this observed motion, with the literature suggesting structures beyond 150 Mpc/h may have considerable contributions. Furthermore, several studies have recently claimed a large volume about the LG is participating in bulk flow, suggesting the existence of coherent structure on larger scales then predicted by the current cosmological paradigm. In this work we attempt to address these issues: how is matter distributed in the local Universe, what structures source the motion of the Local Group, and is there evidence of a bulk flow in excess of predictions from ΛCDM? 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. In this work we compare the predicted velocities from 2M++ to Tully-Fisher and Type Ia Supernova peculiar velocity measurements. We measure a value of β* ≡ f(Ω)/b* = 0.431 ± 0.021, and the root-mean-square density fluctuations of galaxies on an 8 Mpc/h scale to be 0.99 ± 0.04, suggesting f(Ω)σ8 = 0.401 ± 0.024. This value is found to be in strong agreement with measurements from independent methods (including recent analyses of CMB temperature anisotropies). The acceleration of the Local Group arising from 2M++ alone is found to be 540 ± 40 km/s, towards l = 268° ± 4°, b = 38° ± 6°, only 10° out of alignment with the CMB dipole, the best agreement found to date. To account for velocity contributions arising from objects beyond the 2M++ volume, we fit simultaneously for β* and an external dipole in our analysis. We find that such an external velocity model is preferred at the 5.1σ level, with the best fit velocity being 159 ± 23 km/s towards l = 304° ± 11°, b = 6° ± 13°. Finally, we measure the predicted bulk flow of a 50 Mpc/h Gaussian-weighted volume centred on the Local Group. The bulk flow is found to be 230 ± 30 km/s in the direction l = 293° ± 8°, b = 14° ± 10°, in good agreement with predictions from ΛCDM.