High-Excitation OH and H2O Lines in Markarian 231: The Molecular Signatures of Compact Far-Infrared Continuum Sources.

摘要

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. 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, optically thick medium of effective diameter 200-400 pc. In our best-fit model of total luminosity, the observed OH and H2O high-lying lines arise from a luminous region with radius approximately 100 pc.