2010A&A...518L..43G

The Ultra luminous infrared galaxy (ULIRG) Mrk231 reveals up to seven rotational lines of water (H₂O) in emission, including a very high-lying (E_upper=640K) line detected at a 4σ level, within the Herschel/SPIRE wavelength range (190<λ(µm)<640), whereas PACS observations show one H₂O line at 78µm 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=110-180pc) and warm (T_dust=85-95K), accounting for at least 45% of the bolometric luminosity. The high column density, N(H₂O)∼5x10¹⁷cm^–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 78µm 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 (Fisher et al., 2010A&A...518L..41F). Additional PACS/HIFI observations of H₂O lines are required to constrain the kinematics of the nuclear component, as well as the distribution of H₂O relative to the warm dust.