Water Vapor Emission Reveals a Highly Obscured, Star-forming Nuclear Region in the QSO Host Galaxy APM 08279+5255 at z = 3.9

We present the detection of four rotational emission lines of water vapor, from energy levels E _u/k = 101-454 K, in the gravitationally lensed z = 3.9 QSO host galaxy APM 08279+5255. While the lowest H₂ O lines are collisionally excited in clumps of warm, dense gas (density of hydrogen nuclei n_{{\scriptsize H}} =(3.1+/- 1.2)\times 10^6\;cm^{-3}, gas temperature T _g ~ 105 ± 21 K), we find that the excitation of the higher lines is dominated by the intense local infrared radiation field. Since only collisionally excited emission contributes to gas cooling, we conclude that H₂ O is not a significant coolant of the warm molecular gas. Our excitation model requires the radiatively excited gas to be located in an extended region of high 100 μ m opacity (τ₁₀₀ = 0.9 ± 0.2). Locally, such extended infrared-opaque regions are found only in the nuclei of ultraluminous infrared galaxies. We propose a model where the infrared-opaque circumnuclear cloud, which is penetrated by the X-ray radiation field of the QSO nucleus, contains clumps of massive star formation where the H₂ O emission originates. The radiation pressure from the intense local infrared radiation field exceeds the thermal gas pressure by about an order of magnitude, suggesting close to Eddington-limited star formation in these clumps.