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 gal axy Mkn 231 shows OH and H 2 O lines in absorption from energy levels up to 300 K above the ground s tate, and emission in the [O I] 63 mu m and [C II] 158 mu m lines. Our analysis shows that OH and H 2 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 constrai n 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 ( tau 100 mu m = 1 − 2) medium of effective diameter 200-400 pc. In our best-fit mod el of total luminosity L IR , the observed OH and H 2 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 sugges ts that its infrared emission is dominated by the AGN. The derived column densities N (OH) & 10 17 cm − 2 and N (H 2 O) & 6 × 10 16 cm − 2 may indicate 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 ( ∼ 350 pc) starburst region. We show that the [C II] deficit in Mkn 231 is compatible with a high average abundance of C + because of an extreme overall luminosity to gas mass ratio. T herefore, a [C II] deficit may indicate a significant contribution to the luminosity by an AGN, and/or by extremely efficient star formation.