2008ApJ...675..303G

The ISO LWS far-infrared spectrum of the ultraluminous galaxy Mrk 231 shows OH and H₂ O lines in absorption from energy levels up to 300 K above the ground state, and emission in the [O I] 63 µm and [C II] 158 µm lines. Our analysis shows that OH and H₂ O are radiatively pumped by the far-infrared continuum emission of the galaxy. The absorptions in the high-excitation lines require high far-infrared radiation densities, allowing us to constrain the properties of the underlying continuum source. The bulk of the far-infrared continuum arises from a warm (T_dust=70-100 K), optically thick (τ_100µm=1-2) medium of effective diameter 200-400 pc. In our best-fit model of total luminosity L_IR, the observed OH and H₂ O high-lying lines arise from a luminous (L/L_IR∼0.56) region with radius ∼100 pc. The high surface brightness of this component suggests that its infrared emission is dominated by the AGN. The derived column densities N(OH)≳10¹⁷/cm2 and N(H₂O)≳6x10¹⁶/cm2 may indicate X-ray dominated region (XDR) chemistry, although significant starburst chemistry cannot be ruled out. The lower-lying OH, [C II] 158 µm, and [O I] 63 µm lines arise from a more extended (∼350 pc) starburst region. We show that the [C II] deficit in Mrk 231 is compatible with a high average abundance of C⁺ because of an extreme overall luminosity to gas mass ratio. Therefore, a [C II] deficit may indicate a significant contribution to the luminosity by an AGN, and/or by extremely efficient star formation.