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Published September 10, 2012 | Published
Journal Article Open

Constraints on the Faint End of the Quasar Luminosity Function at z ~ 5 in the COSMOS Field


We present the result of our low-luminosity quasar survey in the redshift range of 4.5 ≲ z ≲ 5.5 in the COSMOS field. Using the COSMOS photometric catalog, we selected 15 quasar candidates with 22 < i' < 24 at z ~ 5 that are ~3 mag fainter than the Sloan Digital Sky Survey quasars in the same redshift range. We obtained optical spectra for 14 of the 15 candidates using FOCAS on the Subaru Telescope and did not identify any low-luminosity type-1 quasars at z ~ 5, while a low-luminosity type-2 quasar at z ~ 5.07 was discovered. In order to constrain the faint end of the quasar luminosity function at z ~ 5, we calculated the 1σ confidence upper limits of the space density of type-1 quasars. As a result, the 1σ confidence upper limits on the quasar space density are Φ < 1.33 × 10^(–7) Mpc^(–3) mag^(–1) for –24.52 < M_(1450) < –23.52 and Φ < 2.88 × 10^(–7) Mpc^(–3) mag^(–1) for –23.52 < M_1450 < –22.52. The inferred 1σ confidence upper limits of the space density are then used to provide constraints on the faint-end slope and the break absolute magnitude of the quasar luminosity function at z ~ 5. We find that the quasar space density decreases gradually as a function of redshift at low luminosity (M_1450 ~ –23), being similar to the trend found for quasars with high luminosity (M_1450 < –26). This result is consistent with the so-called downsizing evolution of quasars seen at lower redshifts.

Additional Information

© 2012 American Astronomical Society. Received 2011 July 9; accepted 2012 July 6; published 2012 August 24. We thank the Subaru staff for their invaluable help and all members of the COSMOS team. This work was financially supported in part by the Japan Society for the Promotion of Science (JSPS; grant Nos. 23244031 and 23654068). This work was also partly supported by the FIRST program "Subaru Measurements of Images and Redshifts (SuMIRe)," which was initiated by the Council for Science and Technology Policy (CSTP). K.M. is financially supported by the JSPS through the JSPS Research Fellowship.

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