Herschel observations of water vapour in Markarian 231
  The Ultra Luminous InfraRed Galaxy Mrk 231 reveals up to seven rotational
lines of water (H2O) in emission, including a very high-lying (E_{upper}=640 K)
line detected at a 4sigma level, within the Herschel/SPIRE wavelength range,
whereas PACS observations show one H2O line at 78 microns in absorption, as
found for other H2O lines previously detected by ISO. The absorption/emission
dichotomy is caused by the pumping of the rotational levels by far-infrared
radiation emitted by dust, and subsequent relaxation through lines at longer
wavelengths, which allows us to estimate both the column density of H2O and the
general characteristics of the underlying far-infrared continuum source.
Radiative transfer models including excitation through both absorption of
far-infrared radiation emitted by dust and collisions are used to calculate the
equilibrium level populations of H2O and the corresponding line fluxes. The
highest-lying H2O lines detected in emission, with levels at 300-640 K above
the ground state, indicate that the source of far-infrared radiation
responsible for the pumping is compact (radius=110-180 pc) and warm
(T_{dust}=85-95 K), accounting for at least 45% of the bolometric luminosity.
The high column density, N(H2O)~5x10^{17} cm^{-2}, found in this nuclear
component, is most probably the consequence of shocks/cosmic rays, an XDR
chemistry, and/or an "undepleted chemistry" where grain mantles are evaporated.
A more extended region, presumably the inner region of the 1-kpc disk observed
in other molecular species, could contribute to the flux observed in low-lying
H2O lines through dense hot cores, and/or shocks. The H2O 78 micron line
observed with PACS shows hints of a blue-shifted wing seen in absorption,
possibly indicating the occurrence of H2O in the prominent outflow detected in
OH (Fischer et al., this volume).
