Properties of the diffuse X-ray background in a high-resolution hydrodynamical simulation

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  • 27 July 2005
Abstract
We study the properties of the diffuse X-ray background by using the results of a cosmological hydrodynamical simulation of the concordance LambdaCDM model. The simulation follows gravitational dynamics and gasdynamics, including also a realistic treatment of physical processes like radiative cooling, star formation and supernova feedback. Using its outputs we produce a set of maps of the X-ray emission from the intergalactic medium up to high redshift. We find that the signal in the soft (0.5-2 keV) band is lognormally distributed with a mean intensity of about 4 10^-12 erg s^-1 cm^-2 deg^-2; approximately 40 per cent of the emission is contributed by warm-hot (10^5<T<10^7 K) gas, and 90 per cent comes from structures with z<0.9. Since the spectrum is soft, being provided mostly by intergalactic medium at low temperature, the total mean intensity in the hard (2-10 keV) X-ray band is smaller by a factor of about 4. In order to put constraints on the physical processes included in our simulation, we compare the observational upper limits for the soft X-ray emission from diffuse gas (1.2 +/- 0.3 10^-12 erg s^-1 cm^-2 deg^-2) with our results. For this purpose we remove the contributions from observable virialized objects (groups and clusters of galaxies) from the simulated maps by adopting different detectability criteria which are calibrated by using the characteristics of objects observed by Chandra at intermediate redshifts. We show that the diffuse soft X-ray emission is consistent with the present observational upper limits. However, if future measurements will further reduce by a factor of 2 the level of the unresolved X-ray background, a more efficient feedback mechanism should be required to suppress the soft emission from gas residing within group-sized haloes and filaments.

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