AGN jets versus accretion as reionization sources

Abstract

Context. Cosmic reionization put an end to the dark ages that came after the recombination era. Observations seem to favor the scenario of massive-star photons generated in low-mass galaxies being responsible for the bulk of reionization and, whereas a possible contribution from AGN accretion disks has been widely considered, they are currently thought to have had a minor role in reionization. Aims. We aim to study the possibility of AGN having contributed to reionization not only through their accretion disks, but also through ionizing photons coming from the AGN jets interacting with the intergalactic medium. Methods. We adopt an empirically derived AGN luminosity function at z ' 6, use X-ray observations to correct it for the presence of obscured sources, and estimate the density of jetted AGN. We then use analytical calculations to derive the fraction of jet energy that goes into ionizing photons. Finally, we compute the contribution of AGN jets to the H II volume filling factor at redshifts z ' 15−5. Results. We show that the contribution of the AGN jet lobes to the reionization of the Universe at z ∼ 6 might have been as high as &10% of that of star-forming galaxies, under the most favorable conditions of jetted and obscuration fraction. Conclusions. The contribution of AGN to the reionization, while most likely not dominant, could have been higher than previously assumed, thanks to the radiation originated in the jet lobes.