Ly alpha Constraints on the Nature of the X-Ray--absorbing Medium in Cooling Flow Clusters

Abstract

X-ray spectra of a significant fraction of cooling flow (CF) clusters of galaxies indicate the presence of large columns of ``cold'' absorbing material. The physical nature of the absorbing medium remains a mystery. Searches for H I in this medium using the 21 cm hyperfine structure line yielded null results in most cases. The Lyα absorption cross section is >=10⁷ times larger than that for the 21 cm line, and it can therefore be used as a much more sensitive probe of H I in clusters. This method is applied to the Perseus CF cluster, where a medium resolution (~250 km s^-1) UV spectrum is available. The upper limit on the H I column obtained using Lyα is at least ~2000 smaller than implied by X-ray spectra, indicating that the X-ray absorber is exceedingly devoid of H I. Higher resolution UV spectra with the Hubble Space Telescope may improve the H I column limits by an additional factor of up to ~4000 and thus put severe constraints on the nature of the X-ray-absorbing medium. Most CF clusters have emission-line nebulae at their center, and this method can therefore be applied in a significant number of CF clusters.