Annu. Rev. Astron. Astrophys. 2000. 38: 667-715
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4.9. Active Galactic Nuclei

The number-density of AGN at HDF depths is of great interest because of its connection to the X-ray background and to the re-ionization of the universe at high redshift. [Jarvis & MacAlpine 1998] identified 12 possible candidates with colors and morphologies consistent with AGN at z > 3.5. A similar study by [Conti et al. 1999] found no candidates above z = 3.5 and an upper limit of 20 candidates at lower z. These relatively low number densities support the prevailing view that the UV emission from AGN at high redshifts is insufficient to account for the ionization of the IGM, and they also suggest that black holes do not form with constant efficiency within cold-dark-matter halos [Haiman et al. 1999]. Fainter AGN yet to be identified may lie within the HDF. Based on the properties of many of the optical counterparts to faint ROSAT X-ray sources, [Almaini & Fabian 1997] estimate that roughly 10% of the galaxies in the HDF are likely to be X-ray luminous, narrow-lined AGN. However, few of the optically identified AGN candidates in the HDF-N are detected in the x-rays by the Chandra observatory [Hornschemeier et al. 2000].