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 similar to 1300 K with tau(2) mu m > 2 and apparent mass, M-hot > 10 M-circle dot. Thus, our observations directly detected the optically thick region of hot dust in APM 08279+5255. H I recombination lines of H alpha (0.656 mu m), Pa alpha (1.875 mu m), and Pa beta (1.282 mu m) are clearly detected at 3.2, 6.3, and 9.3 mu m. Simulations with the photoionization models suggest that APM 08279+5255 has broad-line region (BLR) clouds characterized by log n(H) similar to 12-14 for the gas density, log U similar to -2 to -6 for the ionization parameter, and E(B-V) similar to 0.3-0.6 for the BLR. Thus, optically thick emission of hot dust supports an idea on nonspherical distribution of dust near the central source, consistent with the active galactic nuclei model with the dust torus. The temperature of hot dust and flux ratios of these H I lines are similar to those observed in low-redshift quasars. There are significant time variations in the H I lines, which are probably caused by variations in the brightness of the central source.