Introduction

The first unambiguous detection of molecules in galaxies other than the Milky Way was that of OH in absorption toward the nuclear continuum sources in NGC 253 and M82 (Weliachew 1971). Subsequently, all of the species detected in the Milky Way that show strong emission or absorption lines have now been detected in other galaxies. Although extragalactic molecules have been observed in regions of the spectrum other than the radio, notably the vibrational–rotational lines of H2 in the infrared (Thompson, Lebofsky and Rieke 1978), observations remain largely the domain of radioastronomers. Furthermore, the largest fraction of all extragalactic molecular observations have been carried out using the J = 1−0 transition of CO, because it is the easiest transition to detect in most galaxies.

Initial progress in the field was slow, hampered by the simultaneous requirements of large bandwidth, high sensitivity and high angular resolution. The technical requirements were largely overcome in the early 1980s and the field has now burgeoned into one where developments are occurring so fast, on so many fronts that it is impossible in a short review to do justice to all of them. Accordingly, I have chosen a small subset of what I think are among the most interesting recent developments in the study of molecules in other galaxies with the aim of focussing on a few problems of wide astrophysical interest. A good introduction to the field is the comprehensive review by Morris and Rickard (1982).