redshift survey

redshift survey A survey in which the redshifts of large numbers of galaxies are measured. The aim of these surveys may be either to map the distribution of galaxies or to investigate galaxy evolution. Redshift surveys are important because it is very difficult to measure the true distances of all but the closest galaxies. It is relatively easy, however, to measure a galaxy's redshift, and thus determine its velocity of recession. For nearby galaxies, there is a simple relation between the velocity of a galaxy and its distance—the Hubble law—and so astronomers can estimate the distance of a galaxy from its redshift. Over the last decade astronomers have used this technique to map the three‐dimensional distribution of galaxies in the nearby Universe. The most important surveys have been the Two‐Degree Field (2dF) Redshift Survey on the Anglo‐Australian Telescope, in which the redshifts of several hundred thousand galaxies were measured, and the Sloan Digital Sky Survey, in which the redshifts of almost 1 million galaxies were measured. The results show that galaxies do not lie in spherical clusters and groups, as often imagined. Instead, they lie in filaments and sheets (a structure seen in one of the earliest surveys was nicknamed the Great Wall), surrounding voids in which there are very few galaxies. This remarkable structure is often called the cosmic web.

The two surveys mentioned above covered very large areas of sky but contained few galaxies with very high redshifts. Other surveys, such as the Canada–France Redshift Survey (1992–4), carried out with the Canada–France–Hawaii Telescope, and the Gemini Deep Deep Survey carried out with the Gemini North telescope (2002–03), have covered small areas but, by very long exposures, have succeeded in measuring very faint galaxies with redshifts up to 2 or more. At such large redshifts, the concept of distance ceases to have much meaning. For example, at redshift 1 the Universe has doubled in size in the time between the light leaving the galaxy and reaching the telescope. But in these cases astronomers can estimate from the redshift the look‐back time—the time that light has taken to reach us.