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Title:
High-Redshift QSOs in the SWIRE Survey and the z~3 QSO Luminosity Function
Authors:
Siana, Brian; Polletta, Maria del Carmen; Smith, Harding E.; Lonsdale, Carol J.; Gonzalez-Solares, Eduardo; Farrah, Duncan; Babbedge, Tom S. R.; Rowan-Robinson, Michael; Surace, Jason; Shupe, David; Fang, Fan; Franceschini, Alberto; Oliver, Seb
Affiliation:
AA(Spitzer Science Center, California Institute of Technology, 220-6, Pasadena, CA 91125; .; Center for Astrophysics and Space Sciences, University of California, San Diego, CA 92093-0424.), AB(Center for Astrophysics and Space Sciences, University of California, San Diego, CA 92093-0424.), AC(Center for Astrophysics and Space Sciences, University of California, San Diego, CA 92093-0424.), AD(Infrared Processing and Analysis Center, California Institute of Technology, 100-22, Pasadena, CA 91125.), AE(Institute of Astronomy, University of Cambridge, Cambridge CB3 0HA, UK.), AF(Department of Astronomy, Cornell University, Ithaca, NY 14853.), AG(Astrophysics Group, Blackett Laboratory, Imperial College, London SW7 2BW, UK.), AH(Astrophysics Group, Blackett Laboratory, Imperial College, London SW7 2BW, UK.), AI(Spitzer Science Center, California Institute of Technology, 220-6, Pasadena, CA 91125; .), AJ(Spitzer Science Center, California Institute of Technology, 220-6, Pasadena, CA 91125; .), AK(Spitzer Science Center, California Institute of Technology, 220-6, Pasadena, CA 91125; .), AL(Dipartimento di Astronomia, Universita di Padova, I-35122 Padua, Italy.), AM(Astronomy Center, University of Sussex, Falmer, Brighton BN1 9QH, UK.)
Publication:
The Astrophysical Journal, Volume 675, Issue 1, pp. 49-70. (ApJ Homepage)
Publication Date:
03/2008
Origin:
UCP
ApJ Keywords:
Galaxies: Intergalactic Medium, Galaxies: Quasars: General
DOI:
10.1086/527025
Bibliographic Code:
2008ApJ...675...49S

Abstract

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 deg2 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 that they 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 intergalactic medium at z~3.

Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

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