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Water vapor emission reveals a highly obscured, star forming nuclear region in the QSO host galaxy APM08279+5255 at z=3.9

We present the detection of four rotational emission lines of water vapor, from energy levels Eu/k= 101 - 454 K, in the gravitationally lensed z=3.9 QSO host galaxy APM08279+5255. While the lowest H2O lines are collisionally excited in clumps of warm, dense gas (density of hydrogen nuclei n_H=(3.1 +/- 1.2) x 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 H2O 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 micron opacity (tau_100 = 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 H2O 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.