The motion of the Local Group with respect to the 15,000 kilometer per second Abell cluster inertial frame

Abstract

We have measured the velocity of the Local Group with respect to an inertial frame defined by the 119 Abell and Abell, Corwin, & Olowin (ACO) clusters contained within 15,000 km s^-1^. The observations consist of a full-sky peculiar velocity survey with an effective depth ranging from 8000 to 11,000 km s^-1^, depending on how the observations are weighted with redshift. This is the deepest peculiar velocity survey yet conducted. Clusters are selected by heliocentric redshift, and the sample is volume-limited. We use the Hoessel (1980) relationship between the metric luminosities of the brightest clusters galaxies (BCGs) and the slope of their brightness profiles as the distance indicator. The Cousins R-band luminosity within a metric radius of 10 h^-1^ kpc yields a typical distance error of 16% for a single BCG. We test for convergence of the local flow on scales within 10,000 km s^-1^ (i.e., alignment of the Local Group velocity vector with the cosmic microwave background [CMB] dipole), but we find that our sample is not at rest with respect to the CMB. The velocity of the local Group relative to the Abell cluster sample is 561 +/- 284 km s^-1^ toward l = 220^deg^, b = - 28^deg^ (+/-27^deg^), using optimal redshift weighting. This vector is inconsistent with the Local Group absolute space velocity inferred from the CMB dipole anisotropy at 99.99% confidence. An extensive error analysis has been conducted to validate this result. The result is extremely robust and is insensitive to Galactic extinction, velocity biases, sample composition, and geometry. If the CMB dipole is kinematic in origin, then this result implies that the Abell cluster frame itself is moving at 689 +/- 178 km s^-1^ toward l = 343^deg^, b = +52^deg^ (+/-23^deg^), and that the CMB dipole is generated largely by mass concentrations beyond 100 h^-1^ Mpc.