Revealing O VII from stacked X-ray grating spectra of clusters, groups and elliptical galaxies
Abstract
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⊙.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- March 2011
- DOI:
- 10.1111/j.1745-3933.2010.01000.x
- arXiv:
- arXiv:1012.0235
- Bibcode:
- 2011MNRAS.412L..35S
- Keywords:
-
- intergalactic medium;
- X-rays: galaxies: clusters;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- 5 pages, 3 figures, accepted by MNRAS letters