PROBLEMS WITH COLD CLOUDS IN COOLING FLOWS

Some X-ray observations of cooling-flow clusters show soft X-ray absor ption exceeding that expected along the line of sight through our own Galaxy. This absorption appears at the position of the cooling flow an d covers a similar solid angle at the center of the cluster. The infer red absorbing column densities correspond to a hydrogen mass exceeding 10(11) M., prompting suggestions that the absorbing material is conde nsed gas accumulated from the cooling flow. We explore the characteris tics 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 cooli ng-flow clouds can catalyze molecule formation, making them unobservab le at 21 cm, but dusty molecular clouds should radiate detectable, opt ically thick CO rotational lines, which likewise have not been seen. X -ray transient heating of grains prohibits most of the CO from condens ing onto grain surfaces and thus ensures that the CO lines are optical ly thick. Ionized X-ray-absorbing gas would radiate profusely in optic al, UV, or X-ray emission lines. We report limits on Her and [Fe x] 63 74 Angstrom surface brightnesses from deep long-slit spectroscopy that rule out ionized columns thicker than 10(21) cm(-2) and cooler than 1 .5 x 10(6) K. Limits on O VIII Ly alpha do not allow the X-ray-absorbi ng 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 exti nctions inferred from the deficits of background galaxies and quasars counted behind clusters might be consistent with the dust column densi ties inferred from soft X-ray absorption. If dust is the culprit, limi ts on the 100 mu m luminosities of clusters imply that the dust-to-gas ratio must be higher at similar to 1 Mpc, at which large grains can s urvive for longer than 10(9) yr, than in the cores of clusters, where sputtering destroys grains on a much shorter timescale. However, dust at similar to 1 Mpc in quantities sufficient to produce significant so ft X-ray absorption represents a large fraction of the total metal con tent of a cluster. Submillimeter continuum observations should eventua lly determine whether dust is widespread in the intracluster media of clusters of galaxies.