Annu. Rev. Astron. Astrophys. 1982. 20: 517-45
Copyright © 1982 by . All rights reserved

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4.3. OH Absorption-Line Anomalies

In the only galaxies with reported 1612 and 1720 MHz OH data - NGC 4945 (Whiteoak & Gardner 1975a), NGC 253 (Gardner & Whiteoak 1975), and our Galaxy (Whiteoak & Gardner 1975b) - the satellite lines display anomalous intensity ratios that vary with velocity in the same manner in all three galaxies. This curious effect is apparent in the NGC 253 data in Figure 4. At velocities well above the systemic velocity, the 1612-MHz line is either absent or appears in emission, and the 1720-MHz line is in absorption. At velocities well below the systemic velocity, the opposite is true. By contrast, the main lines appear normal; although their profiles are complicated by superimposed emission, they have relative absorption depths at most velocities that are roughly proportional to their relative line strengths. Whiteoak & Gardner (1975a) suggest that the satellite-line anomalies are produced by differential absorption of far-infrared radiation from the nucleus, although their specific model invokes a rather ad hoc velocity field. The idea of infrared pumping to account for the "mirror symmetry" of the satellite-line profiles is attractive, however, and warrants further investigation.