AKARI Near and Mid-Infrared Spectroscopy of APM 08279+5255 at z=3.91

We present rest-frame optical/near-infrared spectra of the gravitationally lensed quasar APM 08279+5255 at z=3.91 that has been taken using the Infrared Camera (IRC) onboard the AKARI infrared satellite. The observed continuum consists of two components; a power-law component dominating optical wavelengths which is the direct light from the central source and thermal emission dominating near-infrared wavelengths which is attributed to the emission from hot dust in the circumnuclear region. The thermal emission well represents optically thick emission by hot dust at T ∼ 1300 K with &tau_2μm > 2. Thus, our observations directly detected the optically thick region of hot dust in APM 08279+5255. HI recombination lines of Hα(0.656 μm), Paα(1.875 μm), and Paβ(1.282 μm) are clearly detected at 3.2, 6.3, and 9.3 μm. Simulations with the photoionization models suggest that APM 08279+5255 has BLR(Broad Line Region) clouds characterized by log n_H ∼ 12 - 14 for the gas density, log U ∼ -2 - -6 for the ionization parameter, and E(B-V)∼ 0.3 - 0.6 for the broad line region. Thus, optically thick emission of hot dust support an idea on non-spherical distribution of dust near the central source, consistent with the Active Galactic Nuclei model with the dust torus.