Discovery of O VII line emitting gas in elliptical galaxies

In the cores of ellipticals, clusters, and groups of galaxies, the gas has a cooling time shorter than 1 Gyr. It is possible to probe cooling flows through the detection of Fe XVII and OVII emission lines, but so far OVII has not been detected in any individual object. The Reflection Grating Spectrometers (RGS) aboard XMM-Newton are currently the only instruments able to detect OVII in extended objects such as elliptical galaxies and galaxy clusters. We searched for evidence of OVII through all the archival RGS observations of galaxy clusters, groups of galaxies, and elliptical galaxies focusing on those with core temperatures below 1 keV. We have discovered OVII resonance (21.6 angstrom) and forbidden (22.1 angstrom) lines for the first time in the spectra of individual objects. OVII was detected at a level higher than three sigma in six elliptical galaxies: M84, M86, M89, NGC1316, NGC4636, and NGC5846. M84, M86, and M89 are members of the Virgo Cluster, the others are central dominant galaxies of groups, and most them show evidence of O VI in UV spectra. We detect no significant trend between the Fe XVII and OVII resonance-to-forbidden line ratios, possibly because of the limited statistics. The observed line ratios < Fe-r/f, O-r/f > = (0.52 +/- 0.02, 0.9 +/- 0.2) indicate that the spectra of all these ellipticals are affected by resonance scattering, suggesting low turbulence. Deeper exposures will help to understand whether the Fe X VII and OVII lines are both produced by the same cooling gas or by multiphase gas. Our OVII luminosities correspond to 0.2-2 M-circle dot yr(-1), which agree with the predictions for ellipticals. Such weak cooling rates would not be detected in clusters because their spectra are dominated by the emission of hotter gas, and owing to their greater distance, the expected OVII line flux would be undetectable.