Problems with Cold Clouds and Cooling Flows

Details Problems with Cold Clouds and Cooling Flows Problems with Cold Clouds and Cooling Flows

Oct 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...452..164v&link_type=abstract

Astrophysical Journal v.452, p.164

Astronomy and Astrophysics

Astronomy

54

Galaxies: Cooling Flows, Galaxies: Intergalactic Medium, Ism: Dust, Extinction, Galaxies: Clusters: General, X-Rays: Galaxies

Scientific paper

Some X-ray observations of cooling-flow clusters show soft X-ray absorption exceeding that expected along the line of sight through our own Galaxy. This absorption appears at the position of the cooling flow and covers a similar solid angle at the center of the cluster. The inferred absorbing column densities correspond to a hydrogen mass exceeding 1011 Msun, prompting suggestions that the absorbing material is condensed gas accumulated from the cooling flow. We explore the characteristics of cold atomic clouds embedded in an X-ray-emitting cooling flow and find that, if they cover the central 100 kpc of the cluster, they should already have been detected in H I 21 cm emission. Dust in cooling-flow clouds can catalyze molecule formation, making them unobservable at 21 cm, but dusty molecular clouds should radiate detectable, optically thick CO rotational lines, which likewise have not been seen. X-ray transient heating of grains prohibits most of the CO from condensing onto grain surfaces and thus ensures that the CO lines are optically thick. Ionized X-ray- absorbing gas would radiate profusely in optical, UV, or X-ray emission lines. We report limits on Hα and [Fe X] 6374 Å surface brightnesses from deep long-slit spectroscopy that rule out ionized columns thicker than 1021 cm-2 and cooler than 1.5 × 106 K. Limits on 0 VIII Lyα do not allow the X-ray-absorbing gas to be at higher temperatures.
One remaining possibility is that dust in the hot intracluster medium absorbs the soft X-rays. The soft X-ray opacity of dust is similar to its optical opacity. Optical extinctions inferred from the deficits of background galaxies and quasars counted behind clusters might be consistent with the dust column densities inferred from soft X-ray absorption. If dust is the culprit, limits on the 100 microns luminosities of clusters imply that the dust-to-gas ratio must be higher at ˜1 Mpc, at which large grains can survive for longer than 109 yr, than in the cores of clusters, where sputtering destroys grains on a much shorter timescale However, dust at ˜1 Mpc in quantities sufficient to produce significant soft X-ray absorption represents a large fraction of the total metal content of a cluster. Submillimeter continuum observations should eventually determine whether dust is widespread in the intracluster media of clusters of galaxies.

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