Properties of Interstellar Medium In Infrared-bright QSOs Probed by [O I] 63 um and [C II] 158 um Emission Lines
Tsai, Chao-Wei; Zhao, Yinghe; Yan, Lin
United States, China
Abstract
We present a study of the interstellar medium (ISM) in the host galaxies of nine QSOs at 0.1 < z < 0.2 with black hole masses of 3× {10}7 {M}⊙ 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}[{{O}{{I}}]63μ {{m}}}/{L}{{FIR}} and {L}[{{C}{{II}}]}/{L}{{FIR}}) are in the ranges of 2.6 × 10-4 to 10-2 and 2.8 × 10-4 to 2 × 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-1) and exceptionally strong [O I] 63 μm emission with {L}[{{O}{{I}}]63μ {{m}}}/{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 cm-3, 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 cm-3 and {10}3\lt {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 cm-3 and {G}0\gt {10}4.
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.