Warm-hot intergalactic medium contribution to baryonic matter
  Hydrodynamical simulations indicate that substantial fraction of baryons in the Universe remains in a diffuse component - Warm-Hot Intergalactic Medium (WHIM). To determine physical properties (spatial distribution, temperature and density) of the WHIM, spatial structure of the soft extended X-ray emission surrounding field galaxies is carefully investigated using the XMM-Newton EPIC/MOS observations. Angular correlations between the galaxy distribution and the soft X-ray background extending over several arcmin are determined. The correlations at large scales result from the clustering of galaxies. At small scales (below ~2 arcmin) the excess of the soft flux is interpreted as the genuine emission from halos of the WHIM surrounding individual galaxies. Bulk parameters of the average WHIM halos associated with galaxies in the sample are estimated. Typical halo has a characteristic radius of ~0.3 Mpc and a mass of 4 - 7 x 10^11 M_sun. The average density of the WHIM in the local universe amounts to 7 - 11 x 10^{-32} g cm^{-3} (Omega_WHIM = 0.7 - 1.2 %). Observations of the X-ray WHIM emission are in good agreement with the numerical simulations, but accuracy of the observational material is insufficient to constrain the theory of WHIM. A series of deep observations of a moderately numerous sample of low redshift galaxies with high resolution instruments of Chandra would significantly improve our estimates of the WHIM parameters. 
Andrzej M. Soltan (Nicolaus Copernicus Astronomical Center, Warsaw)