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 μm and [C II] 158 μm 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 (τ_{100μ 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 ∼ 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 cm⁻² and N(H₂O) ≳ 6 × 10¹⁶ 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 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.