Insights into the location and dynamics of the coolest X-ray emitting gas in clusters of galaxies

TL;DR

This study investigates the coolest X-ray emitting gas in galaxy clusters, revealing its connection to other gas phases and how turbulence and feedback processes influence its properties.

Contribution

It extends previous work by analyzing high-resolution spectra to locate OVII gas and explore its relation to turbulence and cooling suppression in galaxy clusters.

Findings

OVII detected in 11 of 24 objects, mainly in cooler systems.

Resonant scattering suggests subsonic turbulence in OVII-emitting regions.

Charge-exchange processes influence line ratios, indicating interactions between gas phases.

Abstract

We extend our previous study of the cool gas responsible for the emission of OVII X-ray lines in the cores of clusters and groups of galaxies. This is the coolest X-ray emitting phase and connects the 10,000 K H {\alpha} emitting gas to the million degree phase, providing a useful tool to understand cooling in these objects. We study the location of the O VII gas and its connection to the intermediate Fe XVII and hotter O VIII phases. We use high-resolution X-ray grating spectra of elliptical galaxies with strong Fe XVII line emission and detect O VII in 11 of 24 objects. Comparing the O VII detection level and resonant scattering, which is sensitive to turbulence and temperature, suggests that OVII is preferably found in cooler objects, where the FeXVII resonant line is suppressed due to resonant scattering, indicating subsonic turbulence. Although a larger sample of sources and further observations is needed to distinguish between effects from temperature and turbulence, our results are consistent with cooling being suppressed at high turbulence as predicted by models of AGN feedback, gas sloshing and galactic mergers. In some objects the OVII resonant-to-forbidden line ratio is decreased by either resonant scattering or charge-exchange boosting the forbidden line, as we show for NGC 4636. Charge-exchange indicates interaction between neutral and ionized gas phases. The Perseus cluster also shows a high Fe XVII forbidden-to- resonance line ratio, which can be explained with resonant scattering by low-turbulence cool gas in the line-of-sight.