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We present an updated determination of the z ~ 4 QSO luminosity function (QLF), improving the quality of the determination of the faint end of the QLF presented in Glikman et al. (2010). We have observed an additional 43 candidates from our survey sample, yielding one additional QSO at z = 4.23 and increasing the completeness of our spectroscopic follow-up to 48% for candidates brighter than R = 24 over our survey area of 3.76 deg2. We study the effect of using K-corrections to compute the rest-frame absolute magnitude at 1450A compared with measuring M1450 directly from the object spectra. We find a luminosity-dependent bias: template-based K-corrections overestimate the luminosity of low-luminosity QSOs, likely due to their reliance on templates derived from higher luminosity QSOs. Combining our sample with bright quasars from the Sloan Digital Sky Survey and using spectrum-based M1450 for all the quasars, we fit a double-power-law to the binned QLF. Our best fit has a bright-end slope, {alpha} = 3.3pm0.2, and faint-end slope, {beta} = 1.6(+0.8/-0.6). Our new data revise the faint-end slope of the QLF down to flatter values similar to those measured at z ~ 3. The break luminosity, though poorly constrained, is at M* = -24.1(+0.7/-1.9), approximately 1 - 1.5 mag fainter than at z ~ 3. This QLF implies that QSOs account for about half the radiation needed to ionize the IGM at these redshifts.
We have conducted a spectroscopic survey to find faint quasars (-26.0 < M_{1450} < -22.0) at redshifts z=3.8-5.2 in order to measure the faint end of the quasar luminosity function at these early times. Using available optical imaging data from porti
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. Using optical photometric data, we selected 31 quasar candida
We present observations of an optically-faint quasar, RD J114816.2+525339, discovered from deep multi-color observations of the field around the z = 6.42 quasar SDSS J1148+5251. The two quasars have a projected separation of 109 arcsec and both are o
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 SD
Based on results by recent surveys, the number of bright quasars at redshifts z>3 is being constantly revised upwards. Current consensus is that at bright magnitudes ($M_{1450}le -27$) the number densities of such sources could have been underestimat