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redshift survey

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A compilation of the redshifts of a large number of galaxies, or other extragalactic objects, selected according to certain criteria. Such surveys may be used to map out the large-scale structure of the Universe or to investigate galaxy evolution. Redshift surveys are important because it is very difficult to measure true distances of all but the closest galaxies. It is relatively easy, however, to measure a galaxy’s redshift and hence its velocity of recession. For nearby galaxies (redshift 0.1 and less), 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. The distance is only an estimate since galaxies possess peculiar velocities relative to the Hubble flow – any component of this velocity along the radial direction to the galaxy will affect the measured redshift. These peculiar velocities are limited to a few hundred km/s, significantly smaller than the Hubble flow velocities for all but the nearest galaxies. For more distant galaxies, with redshifts much larger than 0.1, the Hubble law breaks down and the redshift–distance relation is used instead.

The two largest redshift surveys to date are the Two-Degree Field Galaxy Redshift Survey (2dFGRS, 1997–2002) which measured nearly 250 000 galaxy redshifts using the Anglo-Australian Telescope, and the Sloan Digital Sky Survey (SDSS, 2000–08) which measured redshifts (in addition to five-band imaging) for almost 1 million galaxies. The results of these and previous redshift surveys show that galaxies do not lie in spherical clusters and groups, but rather lie in filaments and sheets (including the so-called Great Wall) surrounding voids in which there are very few galaxies. This remarkable structure is often called the cosmic web.

Although very large in volume, the 2dFGRS and SDSS surveys are limited to moderate redshifts, with few galaxies beyond a redshift of 0.3. Other surveys have covered much smaller areas of sky but with very long integration times to obtain redshifts of 2 and beyond. Recent examples include the Deep Extragalactic Evolutionary Probe (DEEP) project using the Keck Telescopes and the VIMOS VLT Deep Survey (VVDS) on the European Southern Observatory’s Very Large Telescope. Such surveys are powerful for constraining the evolution of galaxies because the lookback time for high redshift galaxies is a significant fraction of the age of the Universe. For example, it has taken roughly two-thirds of the age of the Universe, around 9 billion years, for light from a galaxy at redshift 2 to reach us. In that time, the Universe has trebled in size.

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