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

Abstract

Abstract 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; LX = 1043–44.5 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.