Discovery of an optically faint quasar at z = 5.70 and implications for the faint end of the quasar luminosity function

We present observations of an optically faint quasar, RD J114816.2+525339 (hereafter RD J1148+5253), discovered from deep multicolor observations of the field around the z = 6.42 quasar SDSS J1148+5251. The two quasars have a projected separation of 109″ and both are outliers in r - z versus z - J color-color space. Keck spectroscopy reveals RD J1148+5253 to be a broad absorption line quasar at z = 5.70. With Z_AB = 23.0, RD J1148+5253 is 3.3 mag fainter than SDSS J1148+5251, making it the faintest quasar known at z > 5.5. This object was identified in a survey of ≈2.5 deg². The implied surface density of quasars at these redshifts and luminosities is broadly consistent with previous extrapolations of the faint end of the quasar luminosity function and supports the idea that active galaxies provide only a minor component of the reionizing ultraviolet flux at these redshifts.