Near-Infrared Photometry of the High-Redshift Quasar RD J030117+002025: Evidence for a Massive Starburst at z = 5.5

With a redshift of z = 5.5 and an optical blue magnitude MB ~ -24.2 mag (~4.5 × 1012 L☉), RD J030117+002025 is the most distant, optically faint (MB > -26 mag) quasar known. Max-Planck Millimeter Bolometer (MAMBO) continuum observations at λ = 1.2 mm (185 μm rest frame) showed that this quasar has a far-infrared (FIR) luminosity comparable to its optical luminosity. We present near-infrared J- and K-band photometry obtained with the Near-Infrared Camera (NIRC) on the Keck I telescope, tracing the slope of the rest-frame ultraviolet (UV) spectrum of this quasar. The observed spectral index is close to the value of αν ~ -0.44 measured in composite spectra of optically bright SDSS quasars. It thus appears that the quasar does not suffer from strong dust extinction, which further implies that its low rest-frame UV luminosity is due to an intrinsically faint active galactic nucleus (AGN). The FIR to optical luminosity ratio is then much larger than that observed for the more luminous quasars, supporting the suggestion that the FIR emission is not powered by the AGN but by a massive starburst.