# The Three Hundred Project: The evolution of galaxy cluster density profiles

## TL;DR

This study confirms that galaxy cluster density profiles are highly self-similar up to redshift 2.5, with their shape established early and evolving through a two-phase accretion process, regardless of baryonic physics inclusion.

## Contribution

It demonstrates the self-similar evolution of galaxy cluster density profiles up to high redshift using hydrodynamical simulations, and links this to halo formation history and accretion phases.

## Key findings

- Median density profiles are in place by z=2.5.
- Relaxed clusters show a shift in scale radius over time.
- Density profile evolution is consistent across different baryonic physics models.

## Abstract

Recent numerical studies of the dark matter density profiles of massive galaxy clusters ($M_{\rm halo} > 10^{15}$M$_{\odot}$) show that their median radial mass density profile remains unchanged up to $z > 1$, displaying a highly self-similar evolution. We verify this by using the data set of the THE THREE HUNDRED project, i.e. 324 cluster-sized haloes as found in full physics hydrodynamical simulations. We track the progenitors of the mass-complete sample of clusters at $z=0$, and find that their median shape is already in place by $z=2.5$. However, selecting a dynamically relaxed subsample ($\sim16$ per cent of the clusters), we observe a shift of the scale radius $r_s$ towards larger values at earlier times. Classifying the whole sample by formation time, this evolution is understood as a result of a two-phase halo mass accretion process. Early-forming clusters -- identified as relaxed today -- have already entered their slow accretion phase, hence their mass growth occurs mostly at the outskirts. Late-forming clusters -- which are still unrelaxed today -- are in their fast accretion phase, thus the central region of the clusters is still growing. We conclude that the density profile of galaxy clusters shows a profound self-similarity out to redshifts $z\sim2.5$. This result holds for both gas and total density profiles when including baryonic physics, as reported here for two rather distinct sub-grid models.