ROSAT evidence for intrinsic oxygen absorption in cooling flow galaxies and groups

The existence of large quantities of gas that have cooled and dropped out o f the hot phase in massive elliptical galaxies, groups, and clusters is the key prediction of the inhomogeneous cooling how scenario. Using spatially resolved, deprojected ROSAT Position Sensitive Proportional Counter (PSPC) spectra of 10 of the brightest cooling flow galaxies and groups with low Ga lactic column densities, we have detected intrinsic absorption over energie s similar to0.4-0.8 keV at the 2 sigma /3 sigma level in half of the sample . Since no intrinsic absorption is indicated for energies below similar to0 .4 keV, the most reasonable model for the absorber is collisionally ionized gas at temperatures T = 10(5-6) K with most of the absorption arising from ionized states of oxygen but with a significant contribution from carbon a nd nitrogen. The soft X-ray emission of this warm gas can also explain the sub-Galactic column densities of cold gas inferred within the central regio ns of most of the systems. (This could not be explained by an absorber comp osed only of dust.) Attributing the absorption to ionized gas reconciles th e large columns of cold H and He inferred from Einstein and ASCA with the l ack of such columns inferred from ROSAT. Within the central similar to 10-20 kpc, where the constraints are most sec ure, the mass of the ionized absorber is consistent with most (perhaps all) of the matter deposited by a cooling flow over the lifetime of the flow. S ince the warm absorber produces no significant H or He absorption, the larg e absorber masses are consistent with the negligible atomic and molecular H inferred from H I and CO observations of cooling hows. It is also found th at if T greater than or similar to 2 x 10(5) K, then the optical and far-ul traviolet emission implied by the warm gas does not violate published const raints. An important theoretical challenge is to understand how the warm te mperature is maintained and how the gas is supported gravitationally, and w e discuss possible solutions to these problems that would require fundament al modification of the standard cooling flow scenario. Finally, we discuss how the prediction of warm ionized gas as the product of mass dropout in th ese and other cooling flows can be verified with new Chandra and X M M obse rvations.