X‐Ray Flux from the Warm‐Hot Intergalactic Medium

The number of detected baryons in the Universe at z<0.5 is much smaller thanpredicted by standard big bang nucleosynthesis and by the detailed observationof the Lyman alpha forest at red-shift z=2. Hydrodynamical simulations indicatethat a large fraction of the baryons today is expected to be in a ``warm-hot''(10^5-10^7K) filamentary gas, distributed in the intergalactic medium. Thisgas, if it exists, should be observable only in the soft X-ray and UV bands.Using the predictions of a particular hydrodynamic model, we simulated theexpected X-ray flux as a function of energy in the 0.1-2 keV band due to theWarm-Hot Intergalactic Medium (WHIM), and compared it with the flux from localand high red-shift diffuse components. Our results show that as much as 20% ofthe total diffuse X-ray background (DXB) in the energy range 0.37-0.925keVcould be due to X-ray flux from the WHIM, 70% of which comes from filaments atredshift z between 0.1 and 0.6. Simulations done using a FOV of 3', comparablewith that of Suzaku and Constellation-X, show that in more than 20% of theobservations we expect the WHIM flux to contribute to more than 20% of the DXB.These simulations also show that in about 10% of all the observations a singlebright filament in the FOV accounts, alone, for more than 20% of the DXB flux.Red-shifted oxygen lines should be clearly visible in these observations.Comment: 19 pages, 6 figure