Introduction

The nature of `dark matter' is widely recognized as a fundamental astrophysical problem (Carr 1994). Evidence for `missing' dark matter exists on all scales from star clusters to the Universe itself. The current trend is to suspect that the dark matter is mostly non-baryonic in nature (Hegyi & Olive 1986). But our current understanding of Big Bang nucleosynthesis indicates that we have only observed about one tenth of the ordinary baryonic matter in the Universe. This has encouraged speculation that the `missing baryonic mass' is in the form of cold gas (Pfenniger, Combes & Martinet 1994) or hot gas (Barcons, Fabian & Rees 1991). In our view, not enough attention has been paid to the possibility that the missing mass is in the form of warm ionized gas (K) smoothly distributed on scales much larger than the optical disks of galaxies. Such gas, whether photoionized or collisionally ionized, could conceivably be detected by the Fabry-Perot `staring' technique. This was the original motivation for a three year campaign with HIFI at the CFHT 3.6m telescope, and TAURUS-2 at the AAT 3.9m and WHT 4.2m telescopes.

We now present a brief summary of results to date. It is premature to assess the likely mass fraction concealed in ionized gas. Instead, we find that detections of ionized gas at large galactocentric distances have important applications to galaxy dynamics and to probing the ambient radiation field.

© Copyright Astronomical Society of Australia 1997