Preheating in the universe suppressing high energy gamma rays from structure formation

Structure formation in the universe can produce high energy gamma rays from shock-accelerated electrons, and this process may be the origin of the extragalactic gamma-ray background (EGRB) as well as a part of the unidentified sources detected by EGRET in the GeV band, if about 5% of the kinetic energy of the shock is going into electron acceleration. However, we point out that the production of gamma rays may be severely suppressed if the collapsing matter has been preheated by external entropy sources at the time of gravitational collapse, as can be inferred from the luminosity–temperature (LT) relation of galaxy clusters and groups. We also make a rough estimate of this effect by a simple model, showing that the EGRB flux may be suppressed by a factor of about 30. Hence structure formation is difficult to be the dominant origin of EGRB if preheating is actually responsible for the observed anomary in the LT relation. The detectable number of gamma-ray clusters is also reduced, but about 5–10 forming clusters should still be detectable by EGRET all sky, and this number is similar to that of the steady and high-latitude unidentified sources in the EGRET catalog. The future GLAST mission should detect 102–103 gamma-ray clusters of galaxies even if the intergalactic medium has been preheated.