Very high redshift X-ray-selected active galactic nuclei in the Chandra Deep Field-North

Deep Chandra X-ray exposures provide an efficient route for locating optically faint active galactic nuclei (AGNs) at high redshifts. We use deep multicolor optical data to search for z > 5 AGNs in the 2 Ms X-ray exposure of the Chandra Deep Field-North. Of the 423 X-ray sources bright enough (z′ < 25.2) for a color-color analysis, at most one lies at z = 5-6, and none at z > 6. The one z > 5 object is spectroscopically confirmed at z = 5.19. Only 31 of the 77 sources with z′ > 25.2 are undetected in the B or V bands at the 2 σ level and could lie at z > 5. There are too few moderate-luminosity AGNs at z = 5-6.5 to ionize the intergalactic medium.