X‐Ray Flux from the Warm‐Hot Intergalactic Medium

Abstract

The number of detected baryons in the universe at z < 0.5 is much smaller than predicted by standard big bang nucleosynthesis and by the detailed observation of the Lyα forest at redshift z = 2. Hydrodynamic simulations indicate that a large fraction of the baryons is expected to be in a "warm-hot" (105-107 K) filamentary gas, distributed in the intergalactic medium. This gas, 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 the X-ray flux as a function of energy in the 0.1-2 keV band due to the warm-hot intergalactic medium (WHIM) and compared it with the flux from other diffuse components. Our results show that as much as 20% of the total diffuse X-ray background (DXB) in the energy range 0.37-0.925 keV could be due to X-rays from the WHIM, 70% of which from filaments at redshift between 0.1 and 0.6. Simulations done using a FOV of 3' show that in more than 20% of the observations 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 single bright filament in the FOV accounts alone for more than 20% of the DXB flux. Redshifted oxygen lines should be clearly visible in these observations. We also investigate the expected angular distribution of the X-ray flux from the WHIM and found a characteristics angular scale of a few arcminutes.