Streaming velocities and the baryon-acoustic oscillation scale

TL;DR

This paper investigates how primordial streaming velocities between baryons and dark matter affect the baryon-acoustic oscillation (BAO) scale, revealing a significant impact on BAO measurements and galaxy formation analysis.

Contribution

It introduces the first comprehensive calculation of streaming velocities' effect on the galaxy correlation function, highlighting the importance of advection and its impact on BAO scale shifts.

Findings

Impact on BAO peak is enhanced by a factor of ~5 compared to previous studies.

Advection causes a significant shift in the observed BAO position.

Streaming velocities can bias BAO scale measurements by about 0.5% if unaccounted for.

Abstract

At the epoch of decoupling, cosmic baryons had supersonic velocities relative to the dark matter that were coherent on large scales. These velocities subsequently slow the growth of small-scale structure and, via feedback processes, can influence the formation of larger galaxies. We examine the effect of streaming velocities on the galaxy correlation function, including all leading-order contributions for the first time. We find that the impact on the BAO peak is dramatically enhanced (by a factor of ~5) over the results of previous investigations, with the primary new effect due to advection: if a galaxy retains memory of the primordial streaming velocity, it does so at its Lagrangian, rather than Eulerian, position. Since correlations in the streaming velocity change rapidly at the BAO scale, this advection term can cause a significant shift in the observed BAO position. If streaming velocities impact tracer density at the 1% level, compared to the linear bias, the recovered BAO scale is shifted by approximately 0.5%. This new effect, which is required to preserve Galilean invariance, greatly increases the importance of including streaming velocities in the analysis of upcoming BAO measurements and opens a new window to the astrophysics of galaxy formation.