Over the past fifteen years our knowledge of the spatial distribution of galaxies has grown tremendously from a very sketchy outline to what is now a fairly detailed map of the local universe. The main reason for this change has been the veritable explosion in the rate at which galaxian redshifts have been measured. Whereas there were fewer than 2000 redshifts known in 1970, the total of available redshifts was more than 30,000 in 1990, and exceeded 100,000 in 1995 (although many of these are as yet unpublished).
The motivation for obtaining thousands of galaxian redshifts is simple: galaxies are the signposts of the universe, marking the locations of large concentrations of mass. The redshift of a galaxy (along with its position on the sky) represents its address - a universal zip code that allows astronomers to locate it in space. This is, of course, an outcome of Hubble's Law and the general expansion of the universe. The measurement of many redshifts allows us to create a 3D map of the universe around us. The details of the distribution of the galaxies are used by cosmologists as they attempt to understand how the universe has evolved to its current state. Since the appearance of the universe today was very likely dictated by conditions that were fixed at an early stage in its evolution, the distribution of galaxies tells us about what was going on in those very early times.
We have known about the expanding universe for decades (Hubble 1929). Why then has it taken so long for astronomers to obtain the tens of thousands of redshifts necessary for mapping the local universe? The answer is simply that the technology was not up to the task until the mid- to late-1970's. Even with the Palomar 5-m telescope, it would take hours to obtain a photographic spectrum in the 1950's for a galaxy that today would require minutes with a CCD spectrograph. The application of image tubes and 1D digital detectors such as Reticons to the measurement of galaxian spectra (e.g., Huchra et al. 1983; Kirshner et al. 1978) started the period of rapid expansion in the numbers of redshifts available. The pace has continued to grow with the subsequent introduction of CCDs, sensitive HI receivers, and multi-object spectrographs.
The aim of this paper is to review the contribution of HI 21-cm observations to the measurement of redshifts, to show how 21-cm observations complement and extend the efforts of the optical observers, and to take a look at what astronomers have learned about the universe through the efforts of those who have painstakingly obtained and analyzed all those galaxian spectra. The authors would like to stress the following caveats: (1) This paper will discuss HI redshift surveys, i.e., projects carried out with the primary goal of obtaining new redshifts of galaxies. All 21-cm observations yield redshift information of course, but it is beyond the scope of this paper to review all extragalactic HI observations! (2) Although optical redshift information will be used throughout, no attempt is made to give a complete review of the status of optical redshift surveys. A recent review of this field is given by Giovanelli & Haynes (1991).