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 less than or similar to z less than or similar to 5.5 in the COSMOS field. Using the COSMOS photometric catalog, we selected 15 quasar candidates with 22 < i' < 24 at z similar to 5 that are similar to 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 similar to 5, while a low-luminosity type-2 quasar at z similar to 5.07 was discovered. In order to constrain the faint end of the quasar luminosity function at z similar to 5, we calculated the 1 sigma confidence upper limits of the space density of type-1 quasars. As a result, the 1 sigma confidence upper limits on the quasar space density are Phi < 1.33 x 10(-7) Mpc(-3) mag(-1) for -24.52 < M-1450 < -23.52 and Phi < 2.88 x 10(-7) Mpc(-3) mag(-1) for -23.52 < M-1450 < -22.52. The inferred 1s 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 similar to 5. We find that the quasar space density decreases gradually as a function of redshift at low luminosity (M-1450 similar to -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.