The space density of high-redshift QSOs in the Great Observatories Origins Deep Survey

We present a sample of 17 high-redshift (3.5 ≲ z ≲ 5.2) quasi-stellar object (QSO) candidates in the 320 arcmin² area of the Great Observatories Origins Deep Survey, selected in the magnitude range 22.45 < z₈₅₀ < 25.25 using deep imaging with the Advanced Camera for Surveys on board the Hubble Space Telescope and the Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory. On the basis of seven spectroscopic and 10 photometric redshifts, we estimate that the final sample will contain between two and four QSOs with 4 < z < 5.2. A dearth of high-redshift moderate-luminosity (M₁₄₅ ≃ -23) QSOs is observed with respect to predictions based on (1) the extrapolation of the z ∼ 2.7 luminosity function (LF), according to a pure luminosity evolution calibrated by the results of the Sloan Digital Sky Survey, and (2) a constant universal efficiency in the formation of supermassive black holes (SMBHs) in dark matter halos. Evidence is gathered in favor of a density evolution of the LF at high redshift and of a suppression of the formation or feeding of SMBHs in low-mass halos.