Lyα constraints on very low luminosity active galactic nuclei

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 Lya and -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 power-law slope of the very faint end of the luminosity function of AGN is shallower than the slope observed at lower redshifts, β₁ < 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 β₁ < 0, at magnitudes fainter than M_B ∼ -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 M_B ≳ is lower than expected from the M_BH-σ relation by one-two orders of magnitude. Extrapolating from reverberation-mapping studies suggests that these black holes would have M_BH = 10 ⁶-10⁷ 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.