A candidate of a type 2 quasi-stellar object at z = 0.9: Large X-ray absorption with a strong broad Hα emission line

Deep hard X-ray and near-infrared observations of a type 2 radio-quiet quasi-stellar object (QSO) candidate at z = 0.9, AX J08494 + 4454, are reported. The 0.5-10 keV Chandra X-Ray Observatory spectrum of AX J08494 + 4454 is hard and is explained well with a power-law continuum absorbed by a hydrogen column density of (2.3 ± 1.1) × 10²³ cm^-2. The 2-10 keV luminosity of the object is estimated to be 7.2 _-2.0 ^+3.6 × 10⁴⁴ ergs s^-1, after correcting the absorption, and reaches hard X-ray luminosities of QSOs. The large X-ray absorption and the large intrinsic luminosity support the original identification of AX J08494 + 4454 as a type 2 radio-quiet QSO. Nevertheless, the deep Subaru/IRCS J-band spectroscopic observation suggests the presence of a strong broad Hα emission line from AX J08494 + 4454. If real, the broad Hα emission line has a velocity width of 9400 ± 1000 km s ^-1, which corresponds to a typical broad Balmer line velocity width of a luminous QSO. The existence of the strong broad Hα line means that the object is not a type 2 QSO but a luminous cousin of a Seyfert 1.9 galaxy in the source-frame optical spectrum. The Balmer decrement of broad lines and the broad Hα emission to the hard X-ray luminosity ratio suggest that the nucleus is affected by dust extinction with an A_v of 1-3 mag in the optical wavelength. Optical colors and faint optical magnitudes of AX J08494 + 4454 are consistent with the model. The estimated amount of dust extinction is much smaller than that expected from the X-ray column density (A_v = 130 ± 60 mag).