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

Next Contents

1. INTRODUCTION

While the study of the distribution of atomic gas in galaxies and our understanding of its significance have progressed far in the last few decades (e.g. the review by van der Kruit & Allen 1978), the complementary investigation of molecular gas in galaxies has gained substantial momentum only recently. The study of extragalactic molecules had two prerequisites: an appreciation of the importance of the molecular cloud component in our Galaxy, and the advances in radio receiver technology required for measurements of the feeble emission from extragalactic molecular clouds. The present extent of observational work on extragalactic molecules can be judged from Table 1, in which the primary references for molecular detections in other galaxies are given. The references have been ordered essentially chronologically, and include both published works and prepublication material made available to us.

In many galaxies, as in our own, the implied quantity of H2 appears to be comparable to that of HI, although the two forms may be distributed quite differently. It is therefore evident that the evolution of the interstellar medium in these galaxies, and the evolution of the galaxies as a whole, are strong functions of where the H2 is located. This is especially true in view of the apparent close association in our Galaxy between molecular clouds and star formation. The primary impetus for current studies of molecular gas in galaxies is thus to determine some or all of the following accessible characteristics: (a) the form of the radial variation of interstellar matter, (b) the mass fraction in molecular clouds at each radius, (c) the relationship (if any) between galaxy type and the molecular cloud abundance and distribution, (d) the relationship (if any) between the degree of activity in galactic nuclei and the presence of a substantial concentration of molecular gas in the nuclei, and (e) the degree of confinement of molecular clouds to spiral arms or other large-scale features. The latter characteristic is related to the lifetimes of molecular clouds by analysis of the maintenance of large-scale patterns in the presence of differential rotation (e.g. Cohen et al. 1980) and by conservation arguments (e.g. Scoville & Hersh 1979).

Table 1. Molecular clouds in galaxies; references d

Galaxies with several species reported
OH HCO CO H2O HCN H2 HCO+ NH3 CH

NGC 224 = M31 - 22 6a,15 - - - - - -
NGC 253 1 3 5 17 13 - 29 25 26b
NGC 598 = M33 - - 37 14 - - - - -
NGC 660 33 - 34 - - - - - -
NGC 1068 = M77 - - 12 - - 18 - - -
NGC 3034 = M82 1 21 5 4b 13 - 24 - -
NGC 3227 33 - 31 - - - - - -
NGC 3504 33 - 34 - - - - - -
NGC 3628 33 - 34 - - - - - -
NGC 4945 2 3 - 23 - - - - 26
NGC 5128 11 9 - - - - 30 - 26
IC 342 - - 19 20 - - - 25 -
LMC 10 8 7 - - - 30 - 26

Galaxies with one species reported
OH: NGC 3079 (38), NGC 5363 (33), NGC 7469 (38)
CO: NGC 157 (36), NGC 185 (35), NGC 891 (40), NGC 2903 (36), NGC 3031 = M81 (15), NGC 3109 (27), NGC 4051 (27 c, 31), NGC 4254 (39), NGC 4303 (34), NGC 4321 (36), NGC 4631 (34), NGC 4736 (37), NGC 4826 (34), NGC 5055 = M63 (6 a), NGC 5194 = M51 (5), NGC 5236 = M83 (5), NGC 5248 (36), NGC 5457 = M101 (32), NGC 6814 (37), NGC 6946 (19), NGC 7793 (28), IC 10 (27), Maffei 2 (16), DDO 126 (27), DOO 133 (27), SMC (41)

a Contradicted by Rickard et al. (1977a) 20. Huchtmeier et al. 1978
b Reported as tentative detection 21. Graham et al. 1978
c Contradicted by Bieging et al. (1981) 22. Cohen et al. 1979
d References 23. Dos Santos & Lepine 1979
1. Weliachew 1971 24. Stark & Wolff 1979
2. Whiteoak & Gardner 1974 25. Martin & Ho 1979
3. Gardner & Whiteoak 1974 26. Whiteoak et al. 1980
4. Andrew et al. 1975 27. Rowan-Robinson et al. 1980
5. Rickard et al. 1975 28. Elmegreen et al. 1980
6. Solomon & de Zafra 1975 29. Rickard & Palmer 1981a
7. Huggins et al. 1975 30. Batchelor et al. 1981
8. Whiteoak & Gardner l976a 31. Bieging et al. 1981
9. Gardner & Whiteoak 1976a 32. Blitz et al. 1981
10. Whiteoak & Gardner 1976c 33. Rickard et al. 1982
11. Gardner & Whiteoak 1976b 34. Rickard et al., in preparation.
12. Rickard et al. 1977a 35. Johnson & Gottesman 1982
13. Rickard et al. 1977b 36. Elmegreen & Elmegreen 1982
14. Churchwell et al. 1977 37. Blitz & Mathieu, private communication
15. Combes et al. 1977a 38. Turner et al., in preparation
16. Rickard et al. 1977c 39. Vietri, private communication
17. Lepine & Dos Santos 1977 40. Solomon, private communication
18. Thompson et al. 1978 41. Israel, private communication
19. Morris & Lo 1978

We summarize the progress achieved so far on these and other problems encountered in the study of extragalactic molecules. For previous brief reviews, the reader is referred to Whiteoak (1978, 1979) and Rickard (1979). Although the best-observed galaxy is our own, we defer to Burton (1976) for a review of that data, and will treat our Galaxy equally with other members of the present sample of galaxies for which we have some knowledge of the molecular cloud distribution. Indeed. one motivation for studying extragalactic systems is to avoid the ambiguities that arise when the observer is located in the galactic plane. For galaxies having small inclination angles, one can determine the molecular distribution directly, without recourse to an assumed rotation curve.

In the following sections, we first bring together the surveys of CO and OH in order to examine the frequency of their occurrence and the types of galaxies that have abundant molecular material. The large-scale distribution of molecular material can be discussed for a number of galaxies for which millimeter-wavelength CO emission has been mapped. In doing so, we present a classification scheme for describing the molecular morphology of galaxies that is based on a differentiation between central regions and disks. The characteristics of these two components are then individually discussed. We conclude with discussions of extragalactic chemistry and masers.

Much of the data reported in this review are observations at millimeter wavelengths, where calibration schemes vary (Kutner & Ulich 1981). We have followed the convention of reporting intensities as atmosphere-absent antenna temperatures, T*A. This scale is the natural result of the chopper-wheel calibration method (Ulich & Haas 1976).

Next Contents