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

7. EXTRAGALACTIC MASERS

Galactic OH and H₂O masers typically have solar system dimensions or less, and thus require VLBI techniques to be resolved, Extragalactic masers are currently unresolvable, but their locations can serve as indicators of the sites of currently active star formation within the overall distribution of molecular material. Because of the inherent weakness of the signals, only a handful of OH and H₂O masers have been detected. Even so, some unexpected results have appeared. Whiteoak & Gardner (1974) and Gardner & Whiteoak (1975) found main-line OH emission within 30" of the nucleus of NGC 253. Unlike typical galactic type I masers, the NGC 253 emission is strongest at 1667 MHz (cf. Figure 4), and is two orders of magnitude stronger than the strongest galactic type I maser. Gardner & Whiteoak (1975) suggest that the maser is amplifying the strong nuclear continuum emission.

A second "supermaser" was seen in M82 (Nguyen-Q-Rieu et al. 1976). Again, 1667-MHz emission predominates, and is an order of magnitude stronger than any galactic maser. Nguyen-Q-Rieu et al. suggest that the maser either lies directly in front of a strong knot in the nonthermal continuum, combining low gain with a high-brightness input, or else it lies in front of the extended nonthermal continuum background and has high gain. VLA observations by Weliachew et al. (in preparation) show that the maser position is centered near the bright nonthermal point source, but shifts with velocity as if more than one source were present.

It seems likely that the strong far-infrared nuclear flux of these galaxies plays a role in pumping the 1667-MHz masers. Inversions of the main lines can be caused by Doppler overlap of the hyperfine components of the far-infrared transitions connecting the ground state to excited rotational states in the J = 1/2 ladder (Litvak 1969). Alternatively, a far-infrared pump that specifically selects the 1667-MHz transition in a medium undergoing expansion when the dust temperature is low (30-50 K; Telesco & Harper 1980) has been proposed by Morris & Bowers (1980) and Bujarrabal et al. 1980). This pump invokes the nonlocal overlap of the hyperfine components of the infrared transitions, and would he an attractive model for the supermasers if large-scale velocity gradients are present in the masering clouds.

H₂O masers are seen in prominent HII regions in nearby spirals (Churchwell et al. 1977, Huchtmeier et al. 1978, 1980). These masers are "normal," in the sense that their luminosities are near the upper end of the luminosity distribution for galactic masers, but less than that of W49, the strongest galactic H₂O maser. On the other hand, H₂O masers with intrinsic brightnesses much greater than W49 are seen toward the central regions of NGC 253 and NGC 4945 (Lepine & Dos Santos 1977, Dos Santos & Lepine 1979). The NGC 253 H₂O supermaser may be associated with the 1667-MHz supermaser, but it is unlikely that they share similar pump mechanisms. The cool, far-infrared radiation that pumps the OH maser is ineffective at exciting the high-energy H₂O maser levels.

Normal OH and H₂O masers are also seen in the LMC, but only in two H II complexes (Johnston et al. 1971, Kaufmann et al. 1977, Scalise & Braz 1981, 1982, Caswell & Haynes 1981, Haynes & Caswell 1981). One of these, N159 (Henize 1956), seems to be the site of a major molecular cloud (Huggins et al. 1975, Whiteoak & Gardner 1976a, b, Israel et al. 1982).