Probing the faint end of the quasar luminosity function at z ∼ 4 in the COSMOS field

H. Ikeda, T. Nagao, K. Matsuoka, Y. Taniguchi, Y. Shioya, J. R. Trump, P. Capak, A. Comastri, M. Enoki, Y. Ideue, Y. Kakazu, A. M. Koekemoer, T. Morokuma, T. Murayama, T. Saito, M. Salvato, E. Schinnerer, N. Z. Scoville, J. D. Silverman

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44 Citations (Scopus)


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.

Original languageEnglish
JournalAstrophysical Journal Letters
Issue number2 PART II
Publication statusPublished - 2011 Feb 20
Externally publishedYes


  • Cosmology: observations
  • Quasars: general
  • Surveys

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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