High-excitation OH and H_2O lines in Markarian 231: the molecular signatures of compact far-infrared continuum sources
Abstract
The far-infrared spectrum of the ultraluminous infrared galaxy Mkn 231 obtained with the Long Wavelength Spectrometer aboard ISO shows high-excitation OH and H_2O lines in absorption in 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 H_2O are radiatively pumped by the far-infrared continuum emission of the galaxy. The observed absorptions in the high-excitation lines require high far-infrared radiation densities, allowing us to constrain the size, temperature, and opacity of the underlying continuum source. The bulk of the far-infrared continuum arises from warm (T_dust=70-100 K), optically thick (tau_100micron=1-2), environments with an effective diameter of 200-400 pc. In our best-fit model, the observed OH line with highest excitation at 65 micron arises from a luminous (L/L_IR~0.56) region with radius of ~100 pc. The high surface brightness of this component suggests that its infrared emission is dominated by the AGN. In this context the observed molecules with derived column densities of N(OH)>~10^{17} cm^{-2} and N(H_2O)>~6x10^{16} cm^{-2} may be tracers of an XDR, although significant starburst chemistry cannot be ruled out. The lower-lying OH lines, as well as the [C II] 158 $\mu$m and [O I] 63 micron lines, are expected to 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 the extreme overall luminosity to gas mass ratio (~500 Lsun/Msun). Thus, we suggest that a low [C II] to far-infrared flux ratio may be an indication of an important contribution to the luminosity by an AGN, and/or by star formation with extreme efficiency.Keywords
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