Herschel observations of water vapour in Markarian 231

Abstract

The Ultra luminous infrared galaxy (ULIRG) Mrk 231 reveals up to seven rotational lines of water (H_2O) in emission, including a very high-lying (E_(upper) = 640 K) line detected at a 4σ level, within the Herschel/SPIRE wavelength range (190 < λ (μm) < 640), whereas PACS observations show one H_2O line at 78 μm in absorption, as found for other H_2O 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 H_2O 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 H_2O and the corresponding line fluxes. The highest-lying H_2O 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 (Tdust = 85–95 K), accounting for at least 45% of the bolometric luminosity. The high column density, N(H_2O) ~ 5×10^(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 H_2O 78 μm line observed with PACS shows hints of a blue-shifted wing seen in absorption, possibly indicating the occurrence of H_2O in the prominent outflow detected in OH (Fischer et al. 2010, A&A, 518, L41). Additional PACS/HIFI observations of H_2O lines are required to constrain the kinematics of the nuclear component, as well as the distribution of H_2O relative to the warm dust.

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