The origin of the extragalactic gamma-ray background (EGB)is one of the fundamental unsolved problems inastrophysics. The EGB could originatefrom either truly diffuse processes or from unresolvedpoint sources. Truly diffuse emission can arise from some processes such asthe annihilation of dark matter,the emission of high energy particles accelerated by intergalactic shocks whichare produced during large scale structure formation, etc.Now, only a small fraction of the extragalactic gamma-ray emission isresolved into point sources.For a sample ofpossible $\gamma$ -ray point sources that have not been detected by theFermi due to their faint fluxes or soft spectra,we can stack a large number of them to improve the statistics.If their fluxes are not too faint, wecan derive their mean flux and photon index by a maximumlikelihood method.Applying the method tothe Australia Telescope 20 GHz Survey sourcesundetected by the LAT, we found that these sources contributeabout 8.4\,\% of the EGB.which are much smaller than the result (17\,\%) of \citet{G10b}.But our method is more directly.Then, we use the method of image stacking to FIRST sources,and find that the FIRST sources undetected by the LAT can contribute about61.4\,\% of the EGB. Considering the flux limit and incompletenessof the sample at the faint limit, and the contribution of the fainterradio subsample to the EGB cannot be ignored,we think that the point sources must contribute most of the EGB.The main contributor of the EGB maybe blazars, non-blazars AGNs,starburst galaxies, and the sources cannot traced by the FIRST survey(including normal galaxies and radio-quiet AGNs).Even though it is difficult to derive the fraction of each populationcontributing to the EGB using ours method alone,it is clear that standard astrophysical scenarios can beinvoked to explain the EGB.
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