Chemical enrichment RGS cluster sample (CHEERS): Constraints on turbulence

Abstract

Context. Feedback from active galactic nuclei, galactic mergers, and sloshing are thought to give rise to turbulence, which may prevent cooling in clusters. Aims. We aim to measure the turbulence in clusters of galaxies and compare the measurements to some of their structural and evolutionary properties. Methods. It is possible to measure the turbulence of the hot gas in clusters by estimating the velocity widths of their X-ray emission lines. The Reflection Grating Spectrometers aboard XMM-Newton are currently the only instruments provided with sufficient effective area and spectral resolution in this energy domain. We benefited from excellent 1.6 Ms new data provided by the Chemical Enrichment RGS cluster Sample (CHEERS) project. Results. The new observations improve the quality of the archival data and allow us to place constraints for some clusters, which were not accessible in previous work. One-half of the sample shows upper limits on turbulence less than 500 km s-1. For several sources, our data are consistent with relatively strong turbulence with upper limits on the velocity widths that are larger than 1000 km s-1. The NGC 507 group of galaxies shows transonic velocities, which are most likely associated with the merging phenomena and bulk motions occurring in this object. Where both low-and high-ionization emission lines have good enough statistics, we find larger upper limits for the hot gas, which is partly due to the different spatial extents of the hot and cool gas phases. Our upper limits are larger than the Mach numbers required to balance cooling, suggesting that dissipation of turbulence may prevent cooling, although other heating processes could be dominant. The systematics associated with the spatial profile of the source continuum make this technique very challenging, though still powerful, for current instruments. In a forthcoming paper we will use the resonant-scattering technique to place lower-limits on the velocity broadening and provide further insights on turbulence. The ASTRO-H and Athena missions will revolutionize the velocity estimates and discriminate between different spatial regions and temperature phases.