The γ-ray background and the age of protogalaxy formation

Abstract

The diffuse γ-ray background which is implied by the current models of protogalaxy formation is considered here in an attempt to set limits to the redshift z_f of that event. This is obtained by observing that cosmic rays from supernovae exploding in protogalaxies produce an isotropic γ-background in excess of the observed values if protogalaxies form too early. Without any special assumptions, the constraints on z_f turn out to be more significant than any other limit previously reported. In fact, for z_f to be >100, one must postulate either a very low cosmic ray production in supernovae, or a very low density of intergalactic gas, that is, <10⁻¹⁰ cm⁻³, or both.