2016ApJ...824..146Z

We present a study of the interstellar medium (ISM) in the host galaxies of nine QSOs at 0.1 z to 3×{10}^{9} {M}_☉ based on the far-IR spectroscopy taken with Herschel Space Observatory. We detect the [O i] 63 µm ([C ii] 158 µm) emission in 6 (8) out of 8 (9) sources. Our QSO sample has far-infrared luminosities ( L_{FIR} ) ∼ several times {10}^{11}L_☉ . The observed line-to- L_{FIR} ratios ( L_{[ØI]63µm}/ L_{FIR} and L_{[CII]}/ L_{FIR} ) are in the ranges of 2.6 x 10^–4 to 10^–2 and 2.8 x 10^–4 to 2 x 10^–3, respectively (including upper limits). These ratios are comparable to the values found in local ULIRGs, but higher than the average value published so far for z\gt 1 IR-bright QSOs. One target, W0752+19, shows an additional broad velocity component (∼720 km/s) and exceptionally strong [O i] 63 µm emission with L_{[ØI]63µm}/ L_{FIR} of 10^–2, an order of magnitude higher than the average value found among local (U)LIRGs. Combining with the analyses of the Sloan Digital Sky Survey optical spectra, we conclude that the [O i] 63 µm emission in these QSOs is unlikely excited by shocks. We infer that the broad [O i] 63 µm emission in W0752+19 could arise from the warm and dense ISM in the narrow-line region of the central active galactic nucleus. Another possible explanation is the existence of a dense gas outflow with {n}_{
H}∼{10}^{4}/cm3, where the corresponding broad [C ii] emission is suppressed. Based on the far-IR [O i] and [C ii] line ratios, we estimate constraints on the ISM density and UV radiation field intensity of {n}_{
H}10^{3.3}/cm3 and {10}^{3}G_{0}10^{4.2} , respectively. These values are consistent with those found in local Seyfert 1 ULIRGs. In contrast, the gas with broad velocity width in W0752+19 has {n}_{
H}10^{4.3}/cm3 and {G}_{0}10^{4} .