The splashback radius and the radial velocity profile of galaxy clusters in IllustrisTNG

¹ Department of Astronomy and Physics, Saint Mary’s University, 923 Robie Street, Halifax B3H3C3, Canada
e-mail: michele.pizzardo@smu.ca
² Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge 02138, USA

Received: 17 November 2023
Accepted: 16 January 2024

Abstract

We used 1697 clusters of galaxies from the TNG300-1 simulation (mass M_200c > 10¹⁴ M_⊙ and redshift range 0.01 ≤ z ≤ 1.04) to explore the physics of the cluster infall region. We used the average radial velocity profile derived from simulated galaxies, v_rad(r), and the average velocity dispersion of galaxies at each redshift, σ_v(r), to explore cluster-centric dynamical radii that characterize the cluster infall region. We revisited the turnaround radius, the limiting outer radius of the infall region, and the radius where the infall velocity has a well-defined minimum. We also explored two new characteristic radii: (i) the point of inflection of v_rad(r) that lies within the velocity minimum, and (ii) the smallest radius where σ_v(r) = |v_rad(r)|. These two, nearly coincident, radii mark the inner boundary of the infall region where radial infall ceases to dominate the cluster dynamics. Both of these galaxy velocity based radii lie within 1σ of the observable splashback radius. The minimum in the logarithmic slope of the galaxy number density is an observable proxy for the apocentric radius of the most recently accreted galaxies, the physical splashback radius. The two new dynamically derived radii relate the splashback radius to the inner boundary of the cluster infall region.