Exciting molecular hydrogen in the central galaxies of cooling flows

Wilman, R. J.; Edge, A. C.; Johnstone, R. M.; Fabian, A. C.; Allen, S. W.; Crawford, C. S.

Abstract

The origin of rovibrational H2 emission in the central galaxies of cooling flow clusters is poorly understood. Here we address this issue using data from our near-infrared spectroscopic survey of 32 of the most line-luminous such systems, presented in the companion paper by Edge et al.

We consider excitation by X-rays from the surrounding intracluster medium (ICM), ultra-violet (UV) radiation from young stars, and shocks. The v = 1-0K-band lines with upper levels within 104 K of the ground state appear to be mostly thermalized (implying gas densities ≳105 cm−3), with the excitation temperature typically exceeding 2000 K, as found earlier by Jaffe, Bremer & van der Werf. Together with the lack of strong v = 2-0 lines in the H-band, this rules out UV radiative fluorescence.

Using the cloudy photoionization code, we deduce that the H2 lines can originate in a population of dense clouds, exposed to the same hot (T∼ 50 000 K) stellar continuum as the lower density gas which produces the bulk of the forbidden optical line emission in the Hα-luminous systems. This dense gas may be in the form of self-gravitating clouds deposited directly by the cooling flow, or may instead be produced in the high-pressure zones behind strong shocks. Furthermore, the shocked gas is likely to be gravitationally unstable, so collisions between the larger clouds may lead to the formation of globular clusters.

Citation

Wilman, R. J., Edge, A. C., Johnstone, R. M., Fabian, A. C., Allen, S. W., & Crawford, C. S. (2002). Exciting molecular hydrogen in the central galaxies of cooling flows. Monthly Notices of the Royal Astronomical Society, 337(1), 63-70. https://doi.org/10.1046/j.1365-8711.2002.05791.x

• Journal Article Type: Article
• Acceptance Date: Jun 20, 2002
• Online Publication Date: Nov 21, 2002
• Publication Date: 2002-11
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Electronic ISSN: 1365-2966
• Publisher: Royal Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 337
• Issue: 1
• Pages: 63-70
• DOI: https://doi.org/10.1046/j.1365-8711.2002.05791.x
• Public URL: https://durham-repository.worktribe.com/output/1624621
• Publisher URL: http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002MNRAS.337...63W&db_key=AST