High-redshift QSOs in the GOODS
  The Great Observatories Origins Deep Survey provides significant constraints on the space density of less luminous QSOs at high redshift, which is particularly important to understand the interplay between the formation of galaxies and super-massive black holes and to measure the QSO contribution to the UV ionizing background. We present the results of a search for high-z QSOs, identified in the two GOODS fields on the basis of deep imaging in the optical (with HST) and X-ray (Chandra), and discuss the allowed space density of QSOs in the early universe. 
S. Cristiani, D. M. Alexander, F. Bauer, W. N. Brandt, E. T.   Chatzichristou, F. Fontanot, A. Grazian, A. Koekemoer, R. A. ...