Wednesday, July 10, 2013

1307.2251 (Daisuke Nagai et al.)

Predicting Merger-Induced Gas Motions in LCDM Galaxy Clusters    [PDF]

Daisuke Nagai, Erwin T. Lau, Camille Avestruz, Kaylea Nelson, Douglas H. Rudd
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 make a prediction of the level of gas motions expected in the LCDM model using hydrodynamical simulations of galaxy cluster formation, focusing on the merger-induced gas motions. We show that the gas velocity dispersion is larger in more massive clusters, but exhibits large scatter due to the diversity in their dynamical states. We show that systems with large gas motions are morphological disturbed, while early forming, relaxed groups show a smaller level of merger-induced gas motions. By analyzing mock deep ASTRO-H observations of simulated clusters, we show that such observations can accurately measure the gas velocity dispersion in the outskirts of nearby relaxed galaxy clusters. ASTRO-H analysis of merging clusters, on the other hand, requires multi-component spectral fitting, which provides rich information about their dynamical states, such as the peculiar velocities and the velocity dispersion of gas within individual sub-clusters. Our analyses indicate that the ASTRO-H mission should be capable of measuring the missing energy associated with internal gas flows as well as enabling unique studies of substructures in galaxy clusters.
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