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 \times 10 ^ { -4 } .
The flux ratio of the O vii to Fe xvii lines is 1 / 4 to 1 / 8 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 \pm 0.6 \hbox { $ \thinspace \mathrm { Z } _ { \odot } $ } .
Although the continuum around the C vi lines is difficult to measure we can similarly estimate that the C/O ratio is 0.9 \pm 0.3 \hbox { $ \thinspace \mathrm { Z } _ { \odot } $ } .