Prospects for Detecting Clusters of Galaxies through X‐Ray Absorption

Abstract

We investigate the feasibility of detecting high-redshift clusters in absorption against background active galactic nuclei (AGNs) with the next generation of X-ray telescopes. The four limiting factors for detecting clusters in absorption are the sensitivity of the X-ray spectrometers, the abundance of X-ray-bright AGNs, the absorption equivalent width, and the sky covering factor of the hot gas in clusters. Based on isothermal and polytropic temperature profiles for the gas in clusters, we compute the absorption equivalent widths of the most abundant heavy elements in clusters with emission-weighted temperatures between 1 keV (typical of groups) and 12 keV (typical of the hottest known clusters). For typical AGN X-ray spectra, we find that the easiest lines to detect in absorption arise from He-like Si and S and Fe XX in groups, He-like Fe in 2-10 keV clusters, and H-like Fe in the hottest clusters. The sky covering factor of the hot gas is then determined as a function of absorption equivalent width for a nonevolving cluster population and for evolving cluster populations in an open (Ω = 0.3) and flat (Ω = 1.0) universe. In an open universe, approximately 5% of the sky is covered by hot gas with an equivalent width greater than 1 eV for He-like Fe absorption. In a high-density universe, the corresponding sky covering factor is only 1%. Based on the X-ray luminosity function of AGNs, we find that there are several thousand AGNs beyond z = 1 and several hundred AGNs beyond z = 2 that are bright enough to produce detectable absorption features in clusters with future X-ray calorimeters. These results show that an observing program of high-redshift, X-ray-bright AGNs will have the added benefit of detecting a significant sample of high-redshift clusters. High-redshift clusters can easily be missed in X-ray imaging surveys, since their X-ray surface brightness can be below the X-ray background.