Revealing O VII from stacked X-ray grating spectra of clusters, groups
  and elliptical galaxies
  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
2.5x10^-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 +- 0.6 Solar.
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 Solar.
