The Origins of the Bulk flow

Abstract

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 Mpc/h, 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 beta = 0.31 pm 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 H0 = 75.0 pm 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 Mpc/h. The amplitude of this externally driven flow increases monotonically with scale, consistent with the influence of a distant, massive overdensity. These findings reinforce the reliability of the CF4 velocity field while calling into question the assumption of a spatially uniform flow generated by external sources. Our results challenge the commonly made hypothesis that the flow in our local volume due to external mass concentrations can be modeled as being spatially uniform.