The contribution of unresolved radio-loud AGN to the extragalactic diffusegamma-ray background

We present a calculation of the blazar contribution to the extragalactic di ffuse gamma-ray background (EGRB) in the EGRET energy range. Our model is b ased on inverse-Compton scattering as the dominant gamma-ray production pro cess in the jets of flat spectrum radio quasars (FSRQs) and BL Lac objects, and on the unification scheme of radio-loud AGN. According to this picture , blazars represent the beamed fraction of the Fanaroff-Riley radio galaxie s (FR galaxies). The observed log N-log S distribution and redshift distribution of both FSR Qs and BL Lacs constrain our model. Depending slightly on the evolutionary behaviour of blazars, we find that unresolved AGN underproduce the intensit y of the extragalactic background radiation. With our model only 20-40 per cent of the extragalactic background emission can be explained by unresolve d blazars if we integrate to a maximum redshift of Z(max)=3. For Z(max)=5, blazars could account for 40-80 per cent of the EGRB. Roughly 70-90 per cen t of the AGN contribution to the EGRB would result from BL Lacs. While the systematic uncertainties in our estimate for the FSRQ contribution appear s mall, in the case of BL Lacs our model parameters are not consistent with t he results from studies in other wavelength regimes, and therefore may have larger systematic uncertainties. Thus we end up with two possibilities, de pending on whether we underpredict or overpredict the BL Lac contribution: either unresolved AGN cannot account for the entire EGRB, or unresolved BL Lacs produce the observed background. We predict a significant flattening of the gamma-ray log N-log S function i n the next two decades of flux below the EGRET threshold.