Direct and reprocessed γ-ray emission of kpc-scale jets in FR I radio galaxies

We discuss the contribution of kiloparsec-scale jets in FR I radio galaxies to the diffuse γ-ray background radiation. The analyzed γ-ray emission comes from inverse-Compton scattering of starlight photon fields by the ultrarelativistic electrons whose synchrotron radiation is detected from such sources at radio, optical and X-ray energies. We find that these objects, under the minimum-power hypothesis (corresponding to a magnetic field of 300 μG in the brightest knots of these jets), can contribute about one percent to the extragalactic γ-ray background measured by EGRET. We point out that this result already indicates that the magnetic fields in kpc-scale jets of low-power radio galaxies are not likely to be smaller than 10 μG on average, as otherwise the extragalactic γ-ray background would be overproduced.