The X-Ray Forest: A New Prediction of Hierarchical Structure Formation Models

We use numerical simulations of structure formation in a cold dark matter model to predict the absorption lines in the soft X-rays produced by heavy elements in the shock-heated intergalactic medium at low redshift. The simulation incorporates a model for heavy-element production in galaxies and the subsequent dispersion of the metals to the intergalactic medium. We analyze in particular absorption lines produced by oxygen and calculate the ionization stage, taking into account the observed X-ray background at the present time. We find that oxygen is fully ionized by the X-ray background in low-density voids and is mostly in the form of O VII and O VIII in the sheets and filamentary regions. Strong absorption lines of O VII and O VIII with equivalent widths W ~ 100 km s-1 are produced in filamentary regions of overdensities of ~100 and temperatures of ~106 K, located in the outskirts of groups and clusters of galaxies. The O VII line at E = 574 eV is generally the strongest one in these systems. Our model predicts that any X-ray source (such as a quasar) should typically show about one O VII absorption line with W > 100 km s-1 in the interval from z = 0 to z = 0.3. These lines could be detected with the upcoming generation of X-ray telescopes, and their origin in intervening systems could be confirmed by the association with groups of galaxies and X-ray-emitting halos near the line of sight at the same redshift. The hot intergalactic medium may be one of the main reservoirs of baryons in the present universe, and the heavy-element X-ray absorption lines offer a promising possibility of detecting this new component in the near future.