Hubble expansion signature on simulated halo density profiles

A path to observing the turnaround radius

¹ Department of Physics and Institute for Theoretical and Computational Physics, University of Crete, GR-70013 Heraklio, Greece
² Institute of Astrophysics, Foundation for Research and Technology – Hellas, Vassilika Vouton, GR-70013 Heraklio, Greece

^⋆ Corresponding authors; gkorkidis@physics.uoc.gr; pavlidou@physics.uoc.gr

Received: 17 April 2024
Accepted: 11 January 2025

Abstract

Context. Density profiles are important tools in galaxy cluster research, offering insights into clusters dynamical states and their relationship with the broader Universe. While these profiles provide valuable information about the matter content of the Universe, their utility in understanding its dark energy component has remained limited due to a lack of tools allowing us to study the transition from cluster portions that are relaxed and infalling, to those that are merging with the Hubble flow.

Aims. In this work we investigate signatures of this transition in stacked density profiles of simulated cluster-sized halos at different redshifts.

Methods. To highlight the Hubble flow around clusters, we used their turnaround radius to normalize stacked simulated density profiles and calculate their logarithmic slope. Then, we complement our analysis by modeling the outer portions of these profiles assuming Gaussian early Universe statistics and the spherical collapse model (SCM) without shell-crossing.

Results. We find the logarithmic slope of median cluster density profiles beyond the turnaround radius – where the Hubble flow dominates – to be Universal and well described by our model. Importantly, we find the slope of the profiles to diverge from the SCM prediction from within the turnaround radius where the actual profiles exhibit caustics which give rise to the splashback feature.

Conclusions. We suggest utilizing this divergence from the spherical collapse model as a method to identify the turnaround radius in stacked cluster density profiles, offering a new perspective on understanding cluster dynamics and their cosmological implications.