• intergalactic medium;
  • X-rays: galaxies: clusters


We stack 4.6 Ms of high spectral resolution XMM–Newton Reflection Grating Spectrometer spectra from galaxy clusters, groups of galaxies and elliptical galaxies. For those objects with a central temperature of less than 1 keV, we detect O vii for the first time, with a probability of false detection of 2.5 × 10−4. The flux ratio of the O vii to Fe xvii lines is one-fourth to one-eighth of the emission expected for isobaric radiative cooling in the absence of heating. There is either a process preventing cooling below 0.5 keV, anomalous O/Fe abundance ratios, absorbing material around the coolest X-ray-emitting gas or non-radiative cooling taking place. The mean N vii emission line is strong in the sub-keV sample. As the ratio of the hydrogenic N and O lines is largely independent of temperature, we measure a mean N/O ratio of 4.0 ± 0.6 Z. Although the continuum around the C vi lines is difficult to measure, we can similarly estimate that the C/O ratio is 0.9 ± 0.3 Z.