AGN predictions for the Hubble Deep Field and the X-ray background

We estimate the number of active galactic nuclei (AGN) among the galaxies in the Hubble Deep Field (HDF). The recent discovery of a class of X-ray-luminous narrow emission line galaxies has provided a possible explanation for the origin of the X-ray background (XRB), although the nature of the activity is still unresolved. By extrapolating the observed X-ray number count distribution to faint optical magnitudes we predict that this AGN population could account for a significant fraction, similar to 10 per cent, of the galaxies in the HDF. In contrast, normal broad-line QSOs are expected to account for no more than similar to 0.1 per cent of the sources at these magnitudes.