|Annu. Rev. Astron. Astrophys. 1982. 20:
Copyright © 1982 by . All rights reserved
Of the panoply of interstellar molecules in our Galaxy, most will eventually be sought in other galaxies. When such work is routine, it will be very interesting to explore the variation of molecular abundances with galaxy type, galactocentric radius, and metallicity, Presently, however, the study of extragalactic chemistry is limited to a few, mostly low-resolution measurements of the handful of molecules listed in Table 1. Here, we discuss the few significant molecules not treated elsewhere in this review.
Formaldehyde was discovered early in the history of the subject (Gardner & Whiteoak 1974). The H2CO lines are mostly weaker analogs of the OH and HI absorption profiles. As an exception, however, Cohen et al. (1979) have reported H2CO absorption toward a position in the disk of M31, attributing it to anomalous absorption in cold dark clouds seen against the 3 K background. With better sensitivity, one may hope to map the dark clouds throughout the disk of M31.
Stark & Wolff (1979) detected HCO+ in M82 at an intensity comparable to that of HCN. They inferred that the constituent clouds of the central source were massive clouds similar to those of the galactic center. Rickard & Palmer (1981a) detected similar emission in NGC 253. After several approximations, they reduced the HCO+-to-CO ratio to a form depending on the two-thirds power of the cosmic-ray ionization rate. Although the relationship is weighted by the particle density of the constituent clouds and depends weakly on the fractional abundance of heavy metals (neither universal constants), one may entertain the prospect of using HCO+ observations to trace the variation of cosmic-ray densities within and among galaxies. Batchelor et al. (1981) have reported HCO+ emission in N159 in the LMC, at an intensity (compared to other species) that is similar to that for galactic clouds. This might mean that the Elmegreen at al. (1980) suggestion that irregulars are CO-poor because of lower cosmic-ray heating is unlikely to be correct in this case.
Martin & Ho (1979) point out that observations of the (1,1) and (2,2) inversion transitions of NH3 could provide a direct measure of the mean temperature of constituent clouds. Because the lines are weak, only IC 342 has yielded both transitions (Ho et al. 1980). The similarity of the two line intensities is characteristic of the warm molecular clouds in the central regions of our Galaxy (Gusten et al. 1981, Morris et al., in preparation).
Given the difficulties of estimating interstellar molecular abundances in our Galaxy, one would be ill-advised to attempt a detailed chemical analysis of the averaged ensemble of clouds observed in other galaxies. Still, based on a comparison of line intensities, the relative abundances in other galaxies appear to be roughly similar to those in our Galaxy. Such comparisons have been made in a few galaxies for CO, OH, HCN, and HCO+ (Rickard et al. 1977a, b, Stark & Wolff 1979, Rickard & Palmer 1981a). Also, the relative line intensities of CH, OH, and H2CO appear to be "normal" in the galaxies observed by Whiteoak et al. (1980), with the possible exception of an overabundance of CH in NGC 5128.