High-excitation oh and H2O lines in Markarian 231:: The molecular signatures of compact far-infrared continuum sources

The ISO LWS far-infrared spectrum of the ultraluminous galaxy Mrk 231 shows OH and H2O lines in absorption from energy levels up to 300 K above the ground state, and emission in the [O I] 63 mu m and [C II] 158 mu m lines. Our analysis shows that OH and H2O 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 (inverted perpendicular 100 mu m = 1-2) medium of effective diameter 200-400 pc. In our best-fit model of total luminosity L-IR, the observed OH and H2O high-lying lines arise from a luminous (L/L-IR similar to 0.56) region with radius similar to 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) greater than or similar to 10(17) cm(-2) and N(H2O) greater than or similar to 6 x 10(16) cm(-2) may indicate X-ray dominated region (XDR) chemistry, although significant starburst chemistry cannot be ruled out. The lower-lying OH, [ C II] 158 mu m, and [ O I] 63 mu m lines arise from a more extended ( similar to 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.