Dynamical signatures of infall around galaxy clusters: a generalized Jeans equation

TL;DR

This paper introduces a generalized Jeans equation that incorporates infall motions and cosmic expansion to accurately describe galaxy cluster dynamics beyond the virial radius, improving upon standard models.

Contribution

It develops a new formalism extending the Jeans equation to include infall and cosmological effects, validated with simulations for better dynamical predictions.

Findings

Generalized Jeans equation accurately predicts velocity dispersion out to 4 virial radii.

Standard Jeans equation overestimates dispersion beyond 2 virial radii.

Model depends on growth rate of virial radius and velocity profile evolution.

Abstract

We study the internal kinematics of galaxy clusters in the region beyond the sphere of virialization. Galaxies around a virialized cluster are infalling towards the cluster centre with a non-zero mean radial velocity. We develop a new formalism for describing the dynamical state of clusters, by generalizing the standard Jeans formalism with the inclusion of the peculiar infall motions of galaxies and the Hubble expansion as well as the contributions due to background cosmology. Using empirical fits to the radial profiles of density, mean radial velocity and velocity anisotropy of both a stacked cluster-mass halo and two isolated halos of a cosmological dark matter only simulation, we verify that our generalized Jeans equation correctly predicts the radial velocity dispersion out to 4 virial radii. We find that the radial velocity dispersion inferred from the standard Jeans equation is accurate up to 2 virial radii, but overestimated by \approx 20% for the stacked halo and by \approx 40% for the isolated halos, in the range \approx 2-3 virial radii. Our model depends on the logarithmic growth rate of the virial radius (function of halo mass or concentration), which we estimate in 7 different ways, and on the departure from self-similarity of the evolution of the peculiar velocity profile in virial units.