|Annu. Rev. Astron. Astrophys. 1982. 20:
Copyright © 1982 by . All rights reserved
The relatively new study of molecular lines in extragalactic objects will be aided by new observational capabilities expected in the coming decade. The large single-dish millimeter-wavelength telescopes, millimeter-wavelength interferometers, and low-noise receivers currently under development are precisely the tools required to pursue the problems discussed here.
Among exciting prospects for future work are the extensions of far-infrared line studies to extragalactic clouds. The coupling of new receiver technologies to airborne telescopes has enabled the detection of a variety of species relevant to understanding the energetics ([C II]; Russell et al. 1980), chemistry ([C I]; Phillips et al. 1980), and excitation (OH; Storey et al. 1981) of molecular clouds. Their application to extragalactic sources has already begun with a search for the 119µ OH lines toward M82 (Watson, private communication).
There is hope that improved VLBI techniques will permit investigators to map extragalactic masers with milliarcsec resolution and to make proper motion studies of the spots within complex maser sources (cf. Genzel et al. 1981). With observations over a long enough time base, these studies could yield independent distances to galaxies, and possibly measurements of their transverse velocities (Genzel & Downes, private communications).
We may also expect the study of extragalactic molecular clouds to be eventually incorporated into gamma-ray astronomy. The interstellar matter of our Galaxy produces ~ 1042 photons s-1 at energies > 100 MeV (Bignami et al. 1979). With sensitivities envisaged for the next generation of gamma-ray telescopes (e.g. EGRET, with a limiting flux ~ 5 x 10-8 photons cm-2 s-1 at E > 20 MeV), only the Local Group galaxies would be accessible at comparable luminosities. But gamma-ray luminosities probably scale as the square of the mass in interstellar matter (Bignami et al. 1975). The large molecular surface densities of some galaxies, coupled with their tendency toward mild nuclear activity, should rank them among the brighter gamma-ray galaxies.
Perhaps the ultimate motivation for determining the molecular cloud distributions within galaxies is that these data (coupled with HI and stellar data and a measure of the star formation rate) provide a basic constraint for models of galaxy evolution. Moreover, information on the past rate of star formation is available in measures of the isotope ratios as functions of galactocentric radius (cf. Wannier 1980). Such ratios are best determined from the radio lines of molecular clouds, and should be measurable over galactic disks with currently planned instrumentation. In addition, studies of the molecular clouds in the centers of energetic galaxies like M82 and NGC 253 should help clarify the "starburst" phenomenon, which seems to be the root of some varieties of galactic activity (Rieke et al. 1980).
We appreciate the contributions of preprints, unpublished data, and enlightening remarks by F.N. Bash, L. Blitz, J.L. Caswell, F. Combes, A.R. Gillespie, E. Greisen, P.T.P. Ho, F.P. Israel, D.W. Johnson, D.A. Kniffen, M. Kutner, J.R.D. Lepine, K.Y. Lo, R.N. Martin, E. Scalise, Jr., N. Scoville, P. Shaver, J. Smith, P. Solomon, A. Stark, L. Weliachew, J.B. Whiteoak, and J. Young.