On the accretion history of galaxy clusters: temporal and spatial distribution

TL;DR

This paper investigates the mass assembly and gas accretion processes of galaxy clusters using cosmological simulations, revealing the dominant role of mergers, environmental influences, and filamentary flows with distinct thermodynamical properties.

Contribution

It introduces a novel multipolar analysis method to study angularly-resolved gas accretion flows and compares thermodynamical properties of different accretion components.

Findings

Major mergers dominate mass growth.

Gas inflows through filaments have lower entropy.

Environmental density correlates with accretion rates.

Abstract

We analyse the results of an Eulerian AMR cosmological simulation in order to quantify the mass growth of galaxy clusters, exploring the differences between dark matter and baryons. We have determined the mass assembly histories (MAHs) of each of the mass components and computed several proxies for the instantaneous mass accretion rate (MAR). The mass growth of both components is clearly dominated by the contribution of major mergers, but high MARs can also occur during smooth accretion periods. We explored the correlations between MARs, merger events and clusters' environments, finding the mean densities in $1 \leq r/R_{200m} \leq 1.5$ to correlate strongly with $\Gamma_{200m}$ in massive clusters which undergo major mergers through their MAH. From the study of the dark matter velocity profiles, we find a strong anticorrelation between the MAR proxies $\Gamma_{200m}$ and $\alpha_{200m}$. Last, we present a novel approach to study the angularly-resolved distribution of gas accretion flows in simulations, which allows to extract and interpret the main contributions to the accretion picture and to assess systematic differences between the thermodynamical properties of each of these contributions using multipolar analysis. We have preliminarily applied the method to the best numerically-resolved cluster in our simulation. Amongst the most remarkable results, we find that the gas infalling through the cosmic filaments has systematically lower entropy compared to the isotropic component, but we do not find a clear distinction in temperature.