Gas rotation and dark matter halo shape in cool-core clusters of galaxies

Abstract

Aims. We study the possibility that the gas in cool-core clusters of galaxies has non-negligible rotation support, the impact of gas rotation on mass estimates from current X-ray observations, and the ability of forthcoming X-ray observatories to detect such rotation. Methods. We present three representative models of massive cool-core clusters with a rotating intracluster medium (ICM) in equilibrium in cosmologically motivated spherical, oblate, or prolate dark matter halos, represented by physical density–potential pairs. In the models, the gas follows a composite-polytropic distribution, and has rotation velocity profiles consistent with current observational constraints and similar to those found in clusters formed in cosmological simulations. We show that the models are consistent with available measurements of the ICM properties of the massive cluster population: the thermodynamic profiles, the shape of the surface brightness distribution, the hydrostatic mass bias, and the broadening of X-ray emitting lines. Using the configuration for the microcalorimeter onboard the XRISM satellite, we generated a set of mock X-ray spectra for our cluster models, which we then analyzed to make predictions about the rotation speed that will be obtained with such an instrument. We then assessed what fraction of the hydrostatic mass bias of our models could be accounted for by detecting the rotation speed with XRISM spectroscopy over the range (0.1 − 1)r500, sampled with three nonoverlapping pointings. Results. Current data leave room for rotating ICM in cool-core clusters, with peaks in the rotation speed as high as 600 km s−1. We show that such rotation, if present, will be detected with upcoming X-ray facilities such as XRISM and that 60 − 70% of the hydrostatic mass bias due to rotation can be accounted for using the line-of-sight velocity measured from X-ray spectroscopy with XRISM, with a residual bias smaller than 3% at an overdensity of 500. In this way, XRISM will allow us to pin down any mass bias of a different origin from the rotation.