Discovery of Water Vapor in the High-Redshift Quasar APM 08279+5255 at
  Z=3.91
  We report a detection of the excited 220-211 rotational transition of
para-H2O in APM 08279+5255 using the IRAM Plateau de Bure interferometer. At z
= 3.91, this is the highest-redshift detection of interstellar water to date.
From LVG modeling, we conclude that this transition is predominantly
radiatively pumped and on its own does not provide a good estimate of the water
abundance. However, additional water transitions are predicted to be detectable
in this source, which would lead to an improved excitation model. We also
present a sensitive upper limit for the HF J = 1 - 0 absorption toward APM
08279+5255. While the face-on geometry of this source is not favorable for
absorption studies, the lack of HF absorption is still puzzling and may be
indicative of a lower fluorine abundance at z = 3.91 compared with the Galactic
ISM.
