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

We extend our previous study of the cool gas responsible for the emission of O $\tiny \text{VII}$ 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 $\tiny \text{VII}$ gas and its connection to the intermediate Fe $\tiny \text{XVII}$ and hotter O $\tiny \text{VIII}$ phases. We use high-resolution X-ray grating spectra of elliptical galaxies with strong Fe $\tiny \text{XVII}$ line emission and detect O $\tiny \text{VII}$ in 11 of 24 objects. Comparing the O $\tiny \text{VII}$ detection level and resonant scattering, which is sensitive to turbulence and temperature, suggests that O $\tiny \text{VII}$ is preferably found in cooler objects, where the Fe $\tiny \text{XVII}$ 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 active galactic nuclei feedback, gas sloshing and galactic mergers. In some objects, the O $\tiny \text{VII}$ 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 $\tiny \text{XVII}$ forbidden-to-resonance line ratio, which can be explained with resonant scattering by low-turbulence cool gas in the line of sight.
This work is based on observations obtained with XMM–Newton, an ESA science mission funded by ESA Member States and USA (NASA). We also acknowledge support from ERC Advanced Grant Feedback 340442 and new data from the awarded XMM–Newton proposal ID 0760870101. YYZ acknowledges support by the German BMWi through the Verbundforschung under grant 50OR1506.