Introduction

This paper traces some lines of thought concerning the nature of dark matter which the authors have been pursuing for the last year or so. For the most part its content follows the talk given by one of us (Mark I) to the July 1998 Meeting of the Astronomical Society of Australia in Adelaide, with some extra material reflecting subsequent developments.

We begin with a brief overview of current activity directed towards identifying dark matter (§2), then focus on a model in which the Galactic dark matter is composed of planetary-mass gas clouds -- a picture that can be drawn directly from the radio-wave lensing of quasars reported by Fiedler et al (1987, 1994). A lensing event (Extreme Scattering Event, or ESE) occurs when a cloud in the Galactic halo crosses the line of sight: radio waves are refracted by an ionised wind which is evaporated from the neutral cloud by UV radiation (§3). The observed event rate implies that the total mass in clouds is comparable to that of the Galactic dark matter.

This inferred population of cold clouds, each of mass

and size AU, suprisingly, does not violate existing observational constraints (§4). The theoretical issues raised by this scenario, discussed in §5, are quite basic: how did the clouds form, and how have they survived? In §6 we consider other techniques for detecting the cloud population. The most promising signatures are optical flashes from cloud collisions, H emission from nearby clouds, and various lensing phenomena. Finally, the outlook for the model is discussed in §7.

© Copyright Astronomical Society of Australia 1997