Cooling of X-ray emitting gas by heat conduction in the center of cooling flow clusters

Free access article

Issue		A&A Volume 422, Number 2, August I 2004


Page(s)		445 - 452
Section		Cosmology (including clusters of galaxies)
DOI		http://dx.doi.org/10.1051/0004-6361:20034415

A&A 422, 445-452 (2004)
DOI: 10.1051/0004-6361:20034415

Cooling of X-ray emitting gas by heat conduction in the center of cooling flow clusters

N. Soker^{1, 2}, E. L. Blanton^{2, 3} and C. L. Sarazin²

¹ Department of Physics, Technion - Israel Institute of Technology, Haifa 32000, Israel
e-mail: soker@physics.technion.ac.il
² Department of Astronomy, University of Virginia, PO Box 3818, Charlottesville, VA 22903-0818, USA
e-mail: [eblanton;sarazin]@virginia.edu
³ Chandra Fellow

(Received 29 September 2003 / Accepted 24 April 2004 )

Abstract
We study the possibility that a large fraction of the gas at temperatures of ~ 10⁷ K in cooling flow clusters cools by heat conduction to lower temperatures, rather than by radiative cooling. We argue that this process, when incorporated into the so-called "moderate cooling flow model", where the effective age of the intracluster medium is much lower than the age of the cluster, reduces substantially the expected X-ray luminosity from gas residing at temperatures of $\la$ 10⁷ K. In this model, the radiative mass cooling rate of gas at ~ 10⁷ K inferred from X-ray observations, which is < $20 \%$ of the mass cooling rates cited in the past, is easily met. The heat conduction is regulated by reconnection between the magnetic field lines in cold (~ 10⁴ K) clouds and the field lines in the intracluster medium. A narrow conduction front is formed, which, despite the relatively low temperature, allows efficient heat conduction from the hot ICM to the cold clouds. The reconnection between the field lines in cold clouds and those in the intracluster medium occurs only when the magnetic field in the ICM is strong enough. This occurs only in the very inner regions of cooling flow clusters, at $r \sim 10{-}30$ kpc. The large ratio of the number of ${\rm H}\alpha$ photons to the number of cooling hydrogen atoms is explained by this scenario.

Key words: X-rays: galaxies: clusters -- galaxies: cooling flows -- galaxies: magnetic fields -- galaxies: clusters: general

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