Almost all of the extragalactic X-ray background (XRB) at 0.25 keV can be accounted for by radio-quiet quasars, allowing us to derive an upper limit of 4 keV cm−2 s−1 sr−1 keV−1 for the remaining background at 0.25 keV. However, the XRB from the gas haloes of groups of galaxies, with gas removal resulting from cooling accounted for, exceeds this upper limit by an order of magnitude if non-gravitational heating is not included. We calculate this using simulations of halo merger trees and realistic gas density profiles, which we require to reproduce the observed gas fractions and abundances of X-ray clusters. In addition, we find that the entire mass range of groups, from ∼5×1012 to ∼1014 M⊙, contributes to the 0.25-keV background in this case.
In a further study, we reduce the luminosities of groups by maximally heating their gas haloes while maintaining the same gas fractions. This reduces the XRB by only a factor of 2 or less. We thus argue that most of the gas associated with groups must be outside their virial radii. This conclusion is supported by X-ray studies of individual groups.
The properties of both groups and X-ray clusters can be naturally explained by a model in which the gas is given excess specific energies of ∼1 keV per particle by non-gravitational heating. With this excess energy, the gas is gravitationally unbound from groups, but recollapses with the formation of a cluster of temperature ≳1 keV. This is similar to a model proposed by Pen, but is contrary to the evolution of baryons described by Cen & Ostriker.
In addition to the soft XRB spectrum, we simulate source counts in two bands, 0.1–0.4 keV and 0.5–2 keV, for comparison with present and future data.