ROSATEvidence for Intrinsic Oxygen Absorption in Cooling Flow Galaxies and Groups

Using spatially resolved, deprojected ROSAT PSPC spectra of 10 of thebrightest cooling flow galaxies and groups with low Galactic column densitieswe have detected intrinsic absorption over energies ~0.4-0.8 keV in half of thesample. Since no intrinsic absorption is indicated for energies below ~0.4 keV,the most reasonable model for the absorber is collisionally ionized gas attemperatures T=10^{5-6} K with most of the absorption arising from ionizedstates of oxygen but with a significant contribution from carbon and nitrogen.The soft X-ray emission of this warm gas can explain the sub-Galactic columndensities of cold gas inferred within the central regions of most of thesystems. Attributing the absorption to ionized gas reconciles the large columnsof cold H and He inferred from EINSTEIN and ASCA with the lack of such columnsinferred from ROSAT. Within the central ~10-20 kpc, where the constraints aremost secure, the estimated mass of the ionized absorber is consistent with most(perhaps all) of the matter deposited by a cooling flow over the lifetime ofthe flow. Since the warm absorber produces no significant H or He absorptionthe large absorber masses are consistent with the negligible atomic andmolecular H inferred from HI and CO observations of cooling flows. It is alsofound that if T > ~2x10^5 K then the optical and UV emission implied by thewarm gas does not violate published constraints. Finally, we discuss how theprediction of warm ionized gas as the product of mass drop-out in these andother cooling flows can be verified with new CHANDRA and XMM observations.(Abridged)Comment: 17 pages (5 figures), Accepted for publication in ApJ, expanded discussion of multiphase spectral models, theoretical implications of warm gas in cooling flows, and the statistical significance of the oxygen absorptio