The Origins of the Bulk flow

TL;DR

The paper investigates the origin of the large-scale bulk flow using the CF4 peculiar velocity catalog, decomposing observed motions into internal and external components relative to a $200\,h^{-1}$ Mpc scale. It introduces a weighted-average, self-consistency test and employs the minimum-variance formalism to extract a best-fit growth parameter $\beta$ and a CF4-specific $H_0$, finding $\beta = 0.31 \pm 0.01$ and $H_0 = 75.9 \pm 0.1$ km s$^{-1}$ Mpc$^{-1}$. The results show that the observed bulk flow is dominated by external mass fluctuations beyond $200\,h^{-1}$ Mpc and that this external contribution is non-uniform, increasing with scale and strongest near the edge of the survey volume, pointing to a distant overdensity rather than a uniform external flow. These findings reinforce the CF4 velocity field's reliability while challenging common modeling assumptions and highlighting the need for deeper surveys to map distant structures and test standard cosmology.

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 200Mpc/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 = 75.9 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 200Mpc/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 assumption that the flow in our local volume due to external mass concentrations can be modeled as being spatially uniform.

Figures (6)

• Figure 1: Weighted averages of the measured peculiar velocities in bins of $v_{int}$ for the best fit value of $H_o$ but varying values of $\beta$. Each bin contains 3,000 objects. The blue points correspond to $\beta= 0.43$, the red points correspond to $\beta=0.31$ (the best fit value), and the green points correspond to $\beta=0.19$ Velocity of the bin is determined by the weighted average of $v_{int}$ values of the objects in the bin. The red line in the figure has a slope of one.
• Figure 2: Weighted averages of the measured peculiar velocities in bins of $v_{int}$ for the best fit value of $\beta$ but varying values of $H_o$. Each bin contains 3,000 objects. The green points correspond to $H_o= 74$km/s/Mpc, the red points correspond to $H_o=75.8$km/s/Mpc (the best fit value), and the blue points correspond to $H_o=78$km/s/Mpc Velocity of the bin is determined by the weighted average of $v_{int}$ values of the objects in the bin. All lines in the figure have a slope of one.
• Figure 3: Weighted averages of the measured peculiar velocities in bins of $v_{int}$ with their uncertainties, for the best fit values of $\beta$ and $H_o$. The velocity of the bin is determined by the weighted average of $v_{int}$ values of the objects in the bin. The line in the figure has a slope of one (this is the same as the red points and line in Fig. \ref{['fig:hvar']}).
• Figure 4: Weighted averages of the measured peculiar velocities in bins of $v_{int}$ and their uncertainties, for objects with $r> 120h^{-1}$Mpc and using the same best fit values of $\beta$ and $H_o$ used in Fig. \ref{['fig:v_int_avg']}. Velocity of the bin is determined by the weighted average of $v_{int}$ values of the objects in the bin. The line in the figure has a slope of one.
• Figure 5: The components ($U_x,-U_y,U_z$) of the bulk flow and their magnitude ($|U|$) calculated using the MV method for different radius volumes. The orange line is the total bulk flow. The blue line is contribution to the bulk flow from velocities generated by mass distributions within 200$h^{-1}$Mpc. The green line is the bulk flow from velocities generated by mass distributions outside of 200$h^{-1}$Mpc.