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

We study the properties of the diffuse X-ray background by using the results of a cosmological hydrodynamical simulation of the concordance Lambda cold dark matter (Lambda CDM) model. The simulation follows gravitational and gas dynamics and includes a treatment of physical processes, such as radiative cooling, star formation and supernova feedback. From the simulation outputs, we produce a set of two-dimensional maps of the intergalactic medium X-ray emission integrated over redshift. We find that the signal in the soft (0.5-2 keV) band is lognormally distributed with a mean intensity of about 4 x 10(-12) erg s(-1) cm(-2) deg(-2); approximately 40 per cent of the emission originates from warm-hot gas ( defined as baryons with 10(5) < T < 10(7) K), and 90 per cent comes from structures at z < 0.9. Since the spectrum is soft, being mostly provided by the 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 constrain the physical processes included in our simulation, we compare our results with the observed upper limit (1.2 +/- 0.3) x 10(-12) erg s(-1) cm(-2) deg(-2) of the soft X-ray emission due to diffuse gas. To this purpose, we remove the contributions of observable extended objects ( groups and clusters of galaxies) from the simulated maps by adopting different detectability criteria which are calibrated on the properties of systems at intermediate redshifts observed by Chandra. We show that the simulated diffuse soft X-ray emission is consistent with the present observed upper limit. However, if future measurements will decrease the level of the unresolved X-ray background by a factor of 2, a more efficient feedback mechanism should be required to suppress the soft emission of the gas residing within filaments and group-size haloes.