The space density of moderate‐luminosity active galaxies at z = 3

We present an estimate of the space density of active galactic nuclei (AGN) at z = 3 . Combining deep X‐ray data with Lyman break galaxy (LBG) colour selection in the rest‐frame ultraviolet makes for highly efficient identification of AGN in a narrow redshift range ( z ∼ 2.5–3.5) . Using Chandra data from the Groth–Westphal Strip (GWS) and the Hubble Deep Field North (HDF‐N), we find a total of 15 X‐ray‐detected LBGs at z ∼ 3 , the majority of which are unlikely to have been identified in blanket follow‐up surveys of X‐ray‐detected objects. We find the comoving space density of moderate‐luminosity AGN (MLAGN; L X = 10 43 – 44.5 erg s −1 ) at z = 3 to be a factor of ∼10 higher than that of the most powerful objects. The available data are consistent with a roughly constant space density of MLAGN from z = 0.5 to 3, and they are also consistent with a mild decline in the space density above z = 1 as predicted by the luminosity‐dependent density evolution models of Ueda et al. This strong AGN activity at z = 3 argues against previous suggestions that the majority of black hole accretion occurs at low redshift. A further implication of our investigation is that, as far as can be determined from current data, the majority of the AGN population at z ∼ 3 is selected by the LBG dropout technique. Although this is sensitive to AGN with an ultraviolet excess arising from an accretion disc, it predominantly identifies star‐forming galaxies. A significant fraction of X‐ray sources seem likely to be hosted by these more typical LBGs, further strengthening the starburst–AGN connection at high redshift.