Determining Distances to Clusters of Galaxies using Resonant X-ray   Emission Lines
  Bright clusters of galaxies can be seen out to cosmological distances, and thus they can be used to derive cosmological parameters. Although the continuum X-ray emission from the intra-cluster gas is optically thin, the optical depth of resonant lines of ions of heavy elements can be larger than unity. In this Letter we study the feasibility of deriving distances to clusters of galaxies by determining the spatial distribution of the intra-cluster gas from X-ray imaging and the optical depth from resonant emission lines (the XREL method). We solve the radiative transfer problem for line scattering in the hot intra-cluster gas using Monte Carlo simulations. We discuss the spatial and spectral resolutions needed to use the XREL method for accurate determination of distances, and hence cosmological parameters, and show that accurate distances will be obtained by applying this technique with the next generation of high resolution X-ray spectrometers. 
S.M. Molnar (1), M. Birkinshaw (2), R.F. Mushotzky (3) ((1) Institute   for Theoretical Physics, University of Zurich, (2) D...