Lyman Alpha Constraints on Very Low Luminosity AGN

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Abstract
Recent surveys have detected Lya emission from z=4.5-6.5 at luminosities as low as 10^41 erg/s. There is good evidence that low numbers of AGN are among observed faint Lya emitters. Combining these observations with an empirical relation between the intrinsic Lya and B-band luminosities of AGN, we obtain an upper limit on the number density of AGN with absolute magnitudes M_B=[-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 powerlaw slope of the very faint end of the luminosity function of AGN is shallower than the slope observed at lower redshifts, beta <1.6, at the 98% confidence level. In fact, we find marginal evidence that the luminosity function rises with luminosity, corresponding to a powerlaw slope beta -20 is lower than expected from the M_BH-sigma relation by one-two orders of magnitude. Extrapolating from reverberation-mapping studies suggests that these black holes would have M_BH=10^6-10^7 Msun. To facilitate the identification of AGN among observed Lya emitters, we derive observational properties of faint AGN in the Lya line, as well as in the X-ray and optical bands.

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