One of the outstanding problems in astronomy today is the nature of the dark matter (DM) that pervades the Universe. Over a wide range of scales, the dynamics of astronomical systems are dominated by this material which only reveals its presence through gravitational effects. This is an extremely important issue, since the solution to many astrophysical problems would be more readily obtained if the form of the DM could be established. For instance, attempts to understand galaxy formation would be simplified if we knew whether the DM is primarily baryonic or non-baryonic and, if non-baryonic, whether it is ``hot'' or ``cold.''
In September of 1991, a workshop on Dark Matter in Galaxies was held at the Space Telescope Science Institute in Baltimore. The aim of the workshop was to look in detail at one aspect of the DM problem. Specifically, participants concentrated on DM in galaxies, and steered away from DM on larger scales and indirect cosmological considerations that have a bearing on the amount and nature of DM. This encouraged an assessment of current observations that pertain directly to DM in galaxies, so that three fundamental questions could be addressed. How much DM is there in galaxies, how is is it distributed, and what is the nature of this DM? Theoretical ideas of direct relevance to these questions were also discussed.
This review is also restricted to the specific topic of DM in galaxies. While much of the material is based on work reported at the workshop, I have also attempted to bring together relevant results from the literature and from recent preprints. Work that was presented at the workshop but which has not appeared elsewhere is referenced by ``DMW'' (Dark Matter Workshop) throughout. The state of the field was reviewed at length by Trimble in 1987, so I have concentrated on significant developments since that time. Other reviewers have discussed specific aspects of DM since that time and these papers are referenced in the relevant sections below. Trimble (1987) and Faber and Gallagher (1979) give a detailed historical perspective of the problem and the broader issues relating to DM. A recent introductory overview of these issues is provided by Tremaine (1992).
I have tried to give values for the dark-to-luminous mass ratio in the various systems of interest, rather than adopting the more conventional approach of quoting a mass-to-light ratio in a given waveband. Part of the motivation is that the gas mass in some galaxies exceeds the stellar mass. In such cases, the quoted luminous mass includes both gas and stars. Many estimates of the amount of DM in galaxies have some dependence on the Hubble constant, H0. Throughout this review, h = H0 / 100 kms-1 Mpc-1.
The plan of this article is to start with DM in the vicinity of the solar neighborhood and work upwards and outwards to larger mass-scales and more distant objects. In Section 2 the controversial question of DM in the disk of the Milky Way is discussed. The halo of our Galaxy is the subject of Section 3, whereas Sections 4 and 5 deal with DM in dwarf spheroidals and dwarf irregulars, respectively. In Section 6, observations of DM in spirals are reviewed. New and promising work on the dark halos of ellipticals is covered in Section 7. The extent of halos around galaxies in groups and clusters is addressed in Section 8, and Section 9 is devoted to the shape of dark halos. In Section 10, laboratory and astrophysical constraints as well as searches for DM in galaxies are reviewed. Other theoretical consequences of these results, particularly for galaxy formation and the nature of the DM are discussed in Section 11. Conclusions are presented in Section 12.