Journal Article

The 6.4-keV fluorescent iron line from cluster cooling flows

E. Churazov, R. Sunyaev, M. Gilfanov, W. Forman and C. Jones

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 297, issue 4, pages 1274-1278
Published in print July 1998 | ISSN: 0035-8711
Published online July 1998 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1046/j.1365-8711.1998.01685.x
The 6.4-keV fluorescent iron line from cluster cooling flows

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The fate of the cooling gas in the central regions of rich clusters of galaxies is not well understood. In one plausible scenario clouds of atomic or molecular gas are formed. However the mass of the cold gas, inferred from measurements of low-energy X-ray absorption, is hardly consistent with the absence of powerful CO or 21-cm emission lines from the cooling flow region. Among the factors which may affect the detectability of the cold clouds are their optical depth, shape and covering fraction. Thus, alternative methods to determine the mass in cold clouds, which are less sensitive to these parameters, are important.

For the inner region of the cooling flow (e.g. within a radius of ∼50–100 kpc) the Thomson optical depth of the hot gas in a massive cooling flow can be as large as ∼ 0.01. Assuming that the cooling time in the inner region is few times shorter than the lifetime of the cluster, the Thomson depth of the accumulated cold gas can be accordingly higher (if most of the gas remains in the form of clouds). The illumination of the cold clouds by the X-ray emission of the hot gas should lead to the appearance of a 6.4-keV iron fluorescent line, with an equivalent width proportional to τT. The equivalent width only weakly depends on the detailed properties of the clouds, e.g. on the column density of individual clouds, as long as the column density is less than a few 1023 cm−2. Another effect also associated exclusively with the cold gas is a flux in the Compton shoulder of bright X-ray emission lines. It also scales linearly with the Thomson optical depth of the cold gas. With the new generation of X-ray telescopes, combining large effective area and high spectral resolution, the mass of the cold gas in cooling flows (and its distribution) can be measured.

Keywords: galaxies: clusters: general; cooling flows; intergalactic medium; X-rays: galaxies

Journal Article.  0 words. 

Subjects: Astronomy and Astrophysics

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