A Differential X-Ray Gunn-Peterson Test Using a Giant Cluster Filament

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Abstract
Using CCD detectors on board the forthcoming Chandra X-Ray Observatory and X-Ray Multimirror Mission, it is possible to devise a measurement of the absolute density of heavy elements in the hypothetical warm gas that is filling intercluster space. This gas may be the largest reservoir of baryonic matter in the universe, but even its existence has not been proved observationally at low redshifts. The proposed measurement would make use of a unique filament of galaxy clusters spanning over 700 h Mpc (0.1 z 0.2) along the line of sight in a small area of the sky in Aquarius. The surface density of Abell clusters there is more than 6 times the sky average. It is likely that the intercluster matter column density is enhanced by a similar factor, making its detection feasible under certain optimistic assumptions about its density and elemental abundances. One can compare photoabsorption depth, mostly in the partially ionized oxygen edges, in the spectra of clusters at different distances along the filament, looking for a systematic increase of depth with the distance. The absorption can be measured by the same detector and through the same Galactic column, hence the differential test. A CCD moderate-energy resolution (ΔE ~ 100 eV) is adequate for detecting an absorption edge at a known redshift.
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