HIGH-REDSHIFT QSOs IN THE SWIRE SURVEY AND THE z～3 QSO LUMINOSITY FUNCTION

摘要

We use a simple optical /infrared (IR) photometric selection of high-redshift QSOs that identifies a Lyman break in the optical photometry and requires a red IR color to distinguish QSOs from common interlopers. The search yields 100 z ～ 3 (U-dropout) QSO candidates with 19 ＜ r' ＜ 22 over 11.7 deg~2 in the ELAIS-N1 (EN1) and ELAIS-N2 (EN2) fields of the Spitzer Wide-area Infrared Extragalactic (SWIRE) Legacy Survey. The z ～ 3 selection is reliable, with spectroscopic follow-up of 10 candidates confirming thatthey are all QSOs at 2.83 ＜ z ＜ 3.44. We find that our z ～ 4 (g'-dropout) sample suffers from both unreliability and incompleteness but present seven previously unidentified QSOs at 3.50 ＜ z ＜ 3.89. Detailed simulations show our z ～ 3 completeness to be ～ 80%-90% from 3.0 ＜ z ＜ 3.5, significantly better than the ～30%-80% completeness of the SDSS at these redshifts. The resulting luminosity function extends 2 mag fainter than SDSS and has a faint-end slope of β = - 1.42 ± 0.15, consistent with values measured at lower redshift. Therefore, we see no evidence for evolution of the faint-end slope of the QSO luminosity function. Including the SDSS QSO sample, we have now directly measured the space density of QSOs responsible for ～70% of the QSO UV luminosity density at z ～ 3. We derive a maximum rate of H I photoionization from QSOs at z ～ 3.2, Γ = 4.8 × 10~(-13) s~(-1) about half of the total rate inferred through studies of the Lyα forest. Therefore, star-forming galaxies and QSOs must contribute comparably to the photoionization of H I in the inter galactic medium at z ～ 3.