The Water Vapor Spectrum of APM 08279+5255: X-Ray Heating and Infrared
  Pumping over Hundreds of Parsecs
  We present the rest-frame 200--320 \mm\ spectrum of the z=3.91 quasar \apm,
obtained with Z-Spec at the Caltech Submillimeter Observatory. In addition to
the \jeight\ to \jthirteen\ 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 XDR model,
assuming L$_{\rm 1-100\,keV}\sim1\times10^{46}\rm\,erg\,s^{-1}$, and that the
gas is distributed over a 550-pc sizescale, per the now-favored $\mu$=4 lensing
model. The total observed cooling in water corresponds to 6.5$\times10^{9}$
\ls, 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 $\sim$50. Using this
scaling, we estimate an average water abundance relative to \hh\ of
1.4$\times10^{-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\ is more than sufficient to pump this massive
reservoir of warm water vapor.
