ABSTRACT. Hydrodynamic simulations predict that low-redshift baryons are predominantly in a warm-hot intergalactic medium (WHIM), which should exhibit absorption and emission lines in the soft X-ray region. The WHIM is predicted to be most dense around clusters of galaxies. We present our XMM-Newton RGS observations of X Comae, an active galactic nucleus behind the Coma Cluster. We detect absorption by Ne IX and O VIII at the redshift of Coma with an equivalent width of 3.3 ± 1.8 and 1.7 ± 1.3 eV, respectively (90% confidence errors or 2.3 and 1.9 σ confidence detections determined from Monte Carlo simulations). The combined significance of both lines is 3.0 σ, again determined from Monte Carlo simulations. The same observation yields a high-statistics EPIC spectrum of the Coma Cluster gas at the position of X Com. We detect emission by Ne IX with a flux of (2.5 ± 1.2) × 10^-8 photons cm^-2 s^-1 arcmin^-2 (90% confidence errors or 3.4 σ confidence detection). These data permit a number of diagnostics to determine the properties of the material causing the absorption and producing the emission. Although a wide range of properties are permitted, values near the midpoint of the range are temperature ~4 × 10⁶ K, density ~6 × 10^-6 cm^-3 corresponding to an overdensity with respect to the mean of ~32, and line-of-sight path length through it ~41(Z/Z)^-1 Mpc, where (Z/Z) is the neon metallicity relative to solar. These are properties that have been predicted of the WHIM, so we conclude that we have detected the WHIM associated with the Coma Cluster.

Subject headings: galaxies: clusters: individual (Coma); intergalactic medium; large-scale structure of universe; quasars: absorption lines

Print publication: Issue 2 (2007 February 1)
Received 2006 March 13, accepted for publication 2006 October 12