The purpose of this study is to explain the ratios of the OH rotational absorption line strengths observed with ISO Long Wavelength Spectrometer (LWS) in the ultraluminous galaxies Arp 220 and Mkn 231. We argue that the measured equivalent widths of the three lines originating from the ground rotational level, the 53, 79, and 119um lines, are not consistent with the simple absorption in a shell of cool gas surrounding a warmer central source of IR continuum: the 79 and 119um lines are by far too weak. We suggest that besides the large-scale clouds absorbing in OH rotational lines, there are in these galaxies compact source(s) of emission in the 79 and 119um lines. With the resolution and sensitivity achieved with the ISO LWS spectrometer, this emission is not resolved from the absorption, but its presence can be inferred from the weakness of the measured absorption line strengths. We hypothesize that the hidden emission occurs in dense gas/dust envelopes of young stars or in the envelope of a single compact source. Higher frequency rotational or ro-vibrational photons emitted by the OH molecules in the inner, warmer part of an envelope, or by a central source of continuum, are converted, further from the source, into lower frequency rotational photons, via fluorescence or weak laser amplification. A simple numerical model of rotational photon scattering in an optically thick envelope supports our hypothesis: the 79 and 119um transitions, as well as the 163um transition (the only one observed in emission, in Arp220) tend to be overheated or inverted in a wide range of model parameters. The source(s) of enhanced emission in these lines may be associated with those of OH megamaser emission. Observations with higher spectral and spatial resolution are needed to detect the supposed hidden emission and confirm or deny this hypothesis.
American Astronomical Society Meeting Abstracts
- Pub Date:
- December 1998