Introduction

Diligent searches in the optical and infrared can narrow the Zone of Avoidance (ZOA) (cf., many contributions in Unveiling Large-Scale Structures behind the Milky Way 1994), but the most opaque regions require searches using the 21-cm line of neutral hydrogen. Unlike at other wavelengths, the opacity of the Milky Way is not a problem at 21 cm (except for extragalactic HI emission near zero velocity, which would be masked by local hydrogen). At the lowest latitudes, HI searches are our only option for mapping large-scale structures.

In this contribution, the results of the feasibility study conducted with the 300-ft telescope at Green Bank, and follow-up study with the Very Large Array will be reviewed (Kerr and Henning 1987, Henning 1992, Henning 1995.) A complete survey of the northern ZOA with the Dwingeloo telescope to 4000 km is ongoing [with collaborators P. Henning and A. Rivers (UNM), R.C. Kraan-Korteweg (Meudon), A. Loan, O. Lahav and D. Lynden-Bell (Cambridge) and W.B. Burton (Leiden)] and the very exciting prospects for the southern Parkes multibeam survey (PI: L. Staveley-Smith (ATNF), with collaborators R. Ekers (ATNF), P. Henning (UNM), R. Kraan-Korteweg (Meudon), A. Green and S. Jurasek (Sydney), and M. Price (ATNF)] will be discussed.

The search method also presents an astrophysical opportunity: the study of a 21-cm-selected sample. Our present concept of a ``normal galaxy'' is based mainly on optical catalogs, on infrared compilations, and on relatively small HI searches. Before a full survey of the ZOA is undertaken, it is useful to understand the sorts of objects we will find, how these differ from samples produced by surveys at other wavelengths, and the impact on our optically-prejudiced notion of normal galaxies.

© Copyright Astronomical Society of Australia 1997