Predicting Merger-Induced Gas Motions in Lambda-CDM Galaxy Clusters

TL;DR

This paper predicts merger-induced gas motions in galaxy clusters using hydrodynamical simulations, highlighting their dependence on cluster mass and morphology, and assesses how upcoming X-ray observations can measure these motions.

Contribution

It provides the first detailed predictions of gas motions in galaxy clusters within the Lambda-CDM model and evaluates the capabilities of the ASTRO-H mission to measure these motions.

Findings

Gas velocity dispersion increases with cluster mass.

Disturbed clusters exhibit larger gas motions than relaxed ones.

ASTRO-H can accurately measure gas motions in relaxed clusters' outskirts.

Abstract

In the hierarchical structure formation model, clusters of galaxies form through a sequence of mergers and continuous mass accretion, which generate significant random gas motions especially in their outskirts where material is actively accreting. Non-thermal pressure provided by the internal gas motions affects the thermodynamic structure of the X-ray emitting intracluster plasma and introduces biases in the physical interpretation of X-ray and Sunyaev-Zeldovich effect observations. However, we know very little about the nature of gas motions in galaxy clusters. The ASTRO-H X-ray mission, scheduled to launch in 2015, will have a calorimeter capable of measuring gas motions in galaxy clusters at the level of <100 km/s. In this work, we predict the level of merger-induced gas motions expected in the Lambda-CDM model using hydrodynamical simulations of galaxy cluster formation. We show that the gas velocity dispersion is larger in more massive clusters, but exhibits a large scatter. We show that systems with large gas motions are morphologically disturbed, while early forming, relaxed groups show a smaller level of gas motions. By analyzing mock ASTRO-H observations of simulated clusters, we show that such observations can accurately measure the gas velocity dispersion out to the outskirts of nearby relaxed galaxy clusters. ASTRO-H analysis of merging clusters, on the other hand, requires multi-component spectral fitting and enables unique studies of substructures in galaxy clusters by measuring both the peculiar velocities and the velocity dispersion of gas within individual sub-clusters.