Shapes of Gas, Gravitational Potential and Dark Matter in Lambda-CDM Clusters

TL;DR

This study analyzes the three-dimensional shapes of intracluster gas, dark matter, and gravitational potential in Lambda-CDM simulated galaxy clusters, revealing how cooling, star formation, and gas motions influence cluster morphology and observable X-ray properties.

Contribution

It provides a detailed comparison of gas, dark matter, and potential shapes in simulations with different physics, linking shape differences to cooling and gas motions, and suggests observational diagnostics.

Findings

Gas in radiative simulations is more spherical outside the core.

Inner gas shape is more triaxial and oblate due to cooling.

X-ray isophote ellipticity can diagnose gas cooling and motions.

Abstract

We present analysis of the three-dimensional shape of intracluster gas in clusters formed in cosmological simulations of the Lambda-CDM cosmology and compare it to the shape of dark matter distribution and the shape of the overall isopotential surfaces. We find that in simulations with radiative cooling, star formation and stellar feedback (CSF), intracluster gas outside the cluster core is more spherical compared to non-radiative (NR) simulations, while in the core the gas in the CSF runs is more triaxial and has a distinctly oblate shape. The latter reflects the ongoing cooling of gas, which settles into a thick oblate ellipsoid as it loses thermal energy. The shape of the gas in the inner regions of clusters can therefore be a useful diagnostic of gas cooling. We find that gas traces the shape of the underlying potential rather well outside the core, as expected in hydrostatic equilibrium. At smaller radii, however, the gas and potential shapes differ significantly. In the CSF runs, the difference reflects the fact that gas is partly rotationally supported. Interestingly, we find that in NR simulations the difference between gas and potential shape at small radii is due to random gas motions, which make the gas distribution more spherical than the equipotential surfaces. Finally, we use mock Chandra X-ray maps to show that the differences in shapes observed in three-dimensional distribution of gas are discernible in the ellipticity of X-ray isophotes. Contrasting the ellipticities measured in simulated clusters against observations can therefore constrain the amount of cooling of the intracluster medium and the presence of random gas motions in cluster cores.