Journal Article

Collisional heating as the origin of filament emission in galaxy clusters*

G. J. Ferland, A. C. Fabian, N. A. Hatch, R. M. Johnstone, R. L. Porter, P. A. M. Van Hoof and R. J. R. Williams

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 392, issue 4, pages 1475-1502
Published in print February 2009 | ISSN: 0035-8711
Published online January 2009 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2008.14153.x
Collisional heating as the origin of filament emission in galaxy clusters*

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It has long been known that photoionization, whether by starlight or other sources, has difficulty in accounting for the observed spectra of the optical filaments that often surround central galaxies in large clusters. This paper builds on the first of this series in which we examined whether heating by energetic particles or dissipative magnetohydrodynamic (MHD) wave can account for the observations. The first paper focused on the molecular regions which produce strong H2 and CO lines. Here we extend the calculations to include atomic and low-ionization regions. Two major improvements to the previous calculations have been made. The model of the hydrogen atom, along with all elements of the H-like iso-electronic sequence, is now fully nl-resolved. This allows us to predict the hydrogen emission-line spectrum including excitation by suprathermal secondary electrons and thermal electrons or nuclei. We show how the predicted H i spectrum differs from the pure-recombination case. The second update is to the rates for H0–H2 inelastic collisions. We now use the values computed by Wrathmall et al. The rates are often much larger and allow the ro–vibrational H2 level populations to achieve a thermal distribution at substantially lower densities than previously thought.

We calculate the chemistry, ionization, temperature, gas pressure and emission-line spectrum for a wide range of gas densities and collisional heating rates. We assume that the filaments are magnetically confined. The gas is free to move along field lines so that the gas pressure is equal to that of the surrounding hot gas. A mix of clouds, some being dense and cold and others hot and tenuous, can exist. The observed spectrum will be the integrated emission from clouds with different densities and temperatures but the same pressure P/k=nT. We assume that the gas filling factor is given by a power law in density. The power-law index, the only free parameter in this theory, is set by matching the observed intensities of infrared H2 lines relative to optical H i lines. We conclude that the filaments are heated by ionizing particles, either conducted in from surrounding regions or produced in situ by processes related to MHD waves.

Keywords: galaxies: clusters: general; galaxies: clusters: individual: NGC 1275; galaxies: clusters: individual: NGC 4696; intergalactic medium; infrared: galaxies

Journal Article.  20447 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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