Warm-hot intergalactic medium contribution to baryonic matter

Context.Hydrodynamical simulations indicate that a substantial fraction of baryons in the universe remains in a diffuse component – warm-hot intergalactic medium (WHIM). Aims.The goal of this paper is to determine the physical properties (spatial distribution, temperature, and density) of the WHIM. Methods.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 result from the large-scale clustering of galaxies. At small angular scales (below ${\sim} 2\arcmin$), the excess of the soft flux is interpreted as the genuine emission from halos of the WHIM surrounding individual galaxies. Results.Bulk parameters of the average WHIM halos associated with galaxies in the sample are estimated. A typical halo has a characteristic radius of ${\sim} 0.3$ Mpc and a mass of $3{-}7\times 10^{11}$ $M_{\odot}$. The average density of the WHIM detected in soft X-rays in the local universe, $\rho_{\rm WHIM}$, amounts to $5 {-} 11\times 10^{-32}$ g cm-3 ($\rho_{\rm WHIM} / \rho_{\rm baryon} = 8 {-} 19$%). The range of uncertainties reflects both the statistical scatter of the investigated correlations and our poor knowledge of the WHIM characteristics (e.g., metallicity). It is possible that the actual uncertainties are larger. Conclusions.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