Not a Copernican observer: biased peculiar velocity statistics in the local Universe

TL;DR

This paper investigates how local large-scale structures bias the measurement of galaxy peculiar velocity statistics, revealing significant deviations caused by nearby clusters like Virgo, especially in shallow surveys, impacting cosmological inferences.

Contribution

It demonstrates the extent of local environment bias on velocity statistics using N-body simulations and quantifies the effects for different survey depths and distributions.

Findings

Local structures distort velocity statistics significantly.

Deviations up to 200% occur in shallow surveys.

Deeper surveys reduce environmental bias substantially.

Abstract

We assess the effect of the local large scale structure on the estimation of two-point statistics of the observed radial peculiar velocities of galaxies. A large N-body simulation is used to examine these statistics from the perspective of random observers as well as "Local Group (LG)-like" observers conditioned to reside in an environment resembling the observed universe within 20 Mpc. The local environment systematically distorts the shape and amplitude of velocity statistics with respect to ensemble-averaged measurements made by a Copernican (random) observer. The Virgo cluster has the most significant impact, introducing large systematic deviations in all the statistics. For a simple "top-hat" selection function, an idealized survey extending to $\sim 160h^{-1}\,{\rm Mpc}$ or deeper is needed to completely mitigate the effects of the local environment. Using shallower catalogues leads to systematic deviations of the order of $50$ to $200\%$ depending on the scale considered. For a flat redshift distribution similar to the one of the CosmicFlows-3 survey, the deviations are even more prominent in both the shape and amplitude at all separations considered $({\stackrel{<}{{}_\sim}} 100h^{-1}\,{\rm Mpc})$. Conclusions based on statistics calculated without taking into account the impact of the local environment should be revisited.