Probing the Faint End of the Quasar Luminosity Function at z~ 4 in the COSMOS Field
Murayama, T.; Taniguchi, Y.; Silverman, J. D.; Salvato, M.; Morokuma, T.; Comastri, A.; Capak, P.; Koekemoer, A. M.; Schinnerer, E.; Scoville, N. Z.; Trump, J. R.; Saito, T.; Nagao, T.; Shioya, Y.; Ideue, Y.; Matsuoka, K.; Ikeda, H.; Enoki, M.; Kakazu, Y.
Japan, United States, Italy, Germany
Abstract
We searched for quasars that are ~3 mag fainter than the SDSS quasars in the redshift range 3.7 <~ z <~ 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 ~ 4. We obtained optical spectra for most of these candidates using FOCAS on the Subaru telescope and identified eight low-luminosity quasars at z ~ 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 ~ 4 has a much shallower faint-end slope (β = -1.67+0.11 -0.17) 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.