SIMBAD references

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.