Turbulence in clusters of galaxies and X-ray line profiles

Abstract

Large-scale bulk motions and hydrodynamic turbulence in the intergalactic gas that fills clusters of galaxies significantly broaden X-ray emission lines. For lines of heavy ions (primarily helium-like and hydrogen-like iron ions), the hydrodynamic broadening is appreciably larger than the thermal broadening. Since clusters of galaxies have a negligible optical depth for resonant scattering in the forbidden and intercombination lines of these ions, these lines are not additionally broadened. At the same time, they are very intense, which allows deviations of the spectrum from the Gaussian spectrum in the line wings to be investigated. The line shape proves to be an important indicator of bulk hydrodynamic processes. Doppler probing of turbulence becomes possible, because the cryogenic detectors of the X-ray observatories now ready for launch and being planned will have a high energy resolution (from 5 eV for ASTRO-E2 to 1–2 eV for Constellation-X and XEUS). We use the spectral representation of a Kolmogorov cascade in the inertial range to calculate the characteristic shapes of radiation lines. Significant deviations in the line profiles from the Gaussian profile (shape asymmetry, additional peaks, sharp breaks in the exponential tails) are expected for large-scale turbulence. The kinematic SZ effect and the X-ray line profiles carry different information about the hydrodynamic velocity distribution in clusters of galaxies and complement each other, allowing the redshift, the peculiar velocity of the cluster, and the bulk velocity dispersion to be separated.