2006/06/30 by Mark Dijkstra, J. Stuart, Stuart Wyithe +1
Physics and Astronomy · #Astrophysical Phenomena and Observations #Astrophysics and Cosmic Phenomena #Galaxies: Formation, Evolution, Phenomena #astro-ph
paper · pdf · doi:10.1111/j.1365-2966.2006.10939.x
published as Mon.Not.Roy.Astron.Soc.372:1575-1584,2006 · 12 pages, 5 figures. MNRAS in Press
arxiv created 2006/09/21 · openalex publication_date 2006/10/05 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/28
Recent surveys have detected Lyα emission from z= 4.5 to 6.5 at luminosities as low as 1041 erg s−1. There is good evidence that low numbers of active galactic nuclei (AGN) are among observed faint Lyα emitters. Combining these observations with an empirical relation between the intrinsic Lyα and B-band luminosities of AGN, we obtain an upper limit on the number density of AGN with absolute magnitudes MB∈[−16, −19] at z= 4.5–6.5. These AGN are up to two orders of magnitude fainter than those discovered in the Chandra Deep Field, resulting in the faintest observational constraints to date at these redshifts. At z= 4.5, the power-law slope of the very faint end of the luminosity function of AGN is shallower than the slope observed at lower redshifts, βl < 1.6, at the 98 per cent confidence level. In fact, we find marginal evidence that the luminosity function rises with luminosity, corresponding to a power-law slope βl < 0, at magnitudes fainter than MB∼−20 (75 per cent confidence level). These results suggest either that accretion on to lower mass black holes is less efficient than on to their more massive counterparts, or that the number of black holes powering AGN with MB≳−20 is lower than expected from the MBH–σ relation by one–two orders of magnitude. Extrapolating from reverberation-mapping studies suggests that these black holes would have MBH= 106–107 M⊙. To facilitate the identification of AGN among observed Lyα emitters, we derive observational properties of faint AGN in the Lyα line, as well as in the X-ray and optical bands.