A Bound Violation on the Galaxy Group Scale: The Turn-Around Radius of NGC 5353/4

TL;DR

This paper presents the first observational evidence of a galaxy group, NGC 5353/4, exceeding the predicted maximum turn-around radius by the LambdaCDM cosmological model, challenging current theoretical expectations.

Contribution

It provides observational evidence of a bound-violation on the galaxy group scale, using velocity profiles and empirical models to compare with LambdaCDM predictions.

Findings

NGC 5353/4's turn-around radius exceeds LambdaCDM predictions.

The observed radius is only marginally consistent with LambdaCDM even without spherical symmetry.

The study introduces a method to estimate turn-around radius from observational data.

Abstract

The first observational evidence for the violation of the maximum turn-around radius on the galaxy group scale is presented. The NGC 5353/4 group is chosen as an ideal target for our investigation of the bound-violation because of its proximity, low-density environment, optimal mass scale, and existence of a nearby thin straight filament. Using the observational data on the line-of-sight velocities and three-dimensional distances of the filament galaxies located in the bound zone of the NGC 5353/4 group, we construct their radial velocity profile as a function of separation distance from the group center and then compare it to the analytic formula obtained empirically by Falco et al. (2014) to find the best-fit value of an adjustable parameter with the help of the maximum likelihood method. The turn-around radius of NGC 5353/4 is determined to be the separation distance where the adjusted analytic formula for the radial velocity profile yields zero. The estimated turn-around radius of NGC 5353/4 turns out to substantially exceed the upper limit predicted by the spherical model based on the LambdaCDM cosmology. Even when the restrictive condition of spherical symmetry is released, the estimated value is found to be only marginally consistent with the LambdaCDM expectation.