The Water Vapor Spectrum of APM 08279+5255: X-Ray Heating and Infrared Pumping over Hundreds of Parsecs

Abstract

We present the rest-frame 200-320 μm spectrum of the z = 3.91 quasar APM 08279+5255, obtained with Z-Spec at the Caltech Submillimeter Observatory. In addition to the J = 8 → 7 to J = 13 → 12 CO rotational transitions which dominate the CO cooling, we find six transitions of water originating at energy levels ranging up to 643 K. Most are first detections at high redshift, and we have confirmed one transition with CARMA. The CO cooling is well described by our X-ray dominated region (XDR) model, assuming L_(1-100 keV) ~ 1 × 10^(46) erg s^(–1), and that the gas is distributed over a 550-pc size scale, as per the now-favored μ = 4 lensing model. The total observed cooling in water corresponds to 6.5 × 109 L_☉, comparable to that of CO. We compare the water spectrum with that of Mrk 231, finding that the intensity ratios among the high-lying lines are similar, but with a total luminosity scaled up by a factor of ~50. Using this scaling, we estimate an average water abundance relative to H_2 of 1.4 × 10^(–7), a good match to the prediction of the chemical network in the XDR model. As with Mrk 231, the high-lying water transitions are excited radiatively via absorption in the rest-frame far-infrared, and we show that the powerful dust continuum in APM 08279+5255 is more than sufficient to pump this massive reservoir of warm water vapor.

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