X-ray Absorption Due to Cold Gas in Cluster Cooling Cores

Abstract

We have calculated the emergent X-ray properties for models of cluster cooling flows including the effects of accumulated cooled material. The opacity of this cooled gas can reduce the overall X-ray luminosity of the cooling flow, and values of Mdot based on these luminosities can underestimate the true value by factors of ~2. We find that accumulated cooled material can produce emergent surface brightness profiles much like those observed even for nearly homogeneous gas distributions. Consequently, much more of the gas may be cooling below X-ray emitting temperatures in the central regions of cooling flows (r < 10 kpc) than one would infer from observed X-ray surface brightness profiles assuming the gas was optically thin. Similarly, the central densities and pressures in cooling flows may have been underestimated. We show that distributed absorption in cooling flows produces a number of observable effects in the spectrum which may allow it to be differentiated from absorption due to gas in our Galaxy. These include a characteristic suppression of the continuum below ~2 keV, absorption features such as a redshifted O K-edge, and diminished intensity of resonance emission lines. Spectra including the effects of intrinsic absorption are not well fit by foreground absorbing models. Attempting to fit such models to the spatially resolved spectra can lead to underestimates of the true absorbing column by factors of 3-20. Fits to integrated spectra of the entire cooling flow region can either underestimate or overestimate the mass of the absorbing gas depending on the specifics of the model. We discuss the potential detection of these effects with AXAF, XMM, and Astro-E.