PROBING THE FAINT END OF THE QUASAR LUMINOSITY FUNCTION AT z ∼ 4 IN THE COSMOS FIELD

We searched for quasars that are similar to 3 mag fainter than the SDSS quasars in the redshift range 3.7 less than or similar to z less than or similar to 4.7 in the COSMOS field to constrain the faint end of the quasar luminosity function (QLF). Using optical photometric data, we selected 31 quasar candidates with 22 < i' < 24 at z similar to 4. We obtained optical spectra for most of these candidates using FOCAS on the Subaru telescope and identified eight low-luminosity quasars at z similar to 4. In order to derive the QLF based on our spectroscopic follow-up campaign, we estimated the photometric completeness of our quasar survey through detailed Monte Carlo simulations. Our QLF at z similar to 4 has a much shallower faint-end slope (beta = -1.67(-0.17)(+ 0.11)) than that obtained by other recent surveys in the same redshift. Our result is consistent with the scenario of downsizing evolution of active galactic nuclei inferred by recent optical and X-ray quasar surveys at lower redshifts.