The Origins of the Bulk flow

We analyze the origin of the large-scale bulk flow using the CosmicFlows-4 (CF4) peculiar-velocity catalog. We decompose the observed motions into internal components, generated by mass fluctuations within 200 h − 1 Mpc, and external ones arising from structures beyond this volume. A weighted-average technique is developed to test the model’s self-consistency while minimizing the impact of non-Gaussian distance errors. The CF4 velocities show excellent agreement with the predicted internal field, yielding β = 0.31 ± 0.01 . We also determine that the value of the Hubble constant that should be used for calculating peculiar velocities from the CF4 to be H 0 = 75.9 ± 0.1 km s Mpc, consistent with CF4 calibrations. Using the minimum-variance formalism, we further separate the bulk flow into its internal and external contributions and find that the observed large-scale bulk flow is dominated by sources beyond 200 h − 1 Mpc. The amplitude of this externally driven flow increases monotonically with scale, consistent with the influence of a distant, massive overdensity. We model this overdensity and determine its most likely mass and distance. Our results challenge the commonly made assumption that the flow in our local volume due to external mass concentrations can be modeled as being spatially uniform.