Inastronomy, aredshift survey is asurvey of a section of the sky to measure theredshift ofastronomical objects: usually galaxies, but sometimes other objects such as galaxy clusters or quasars. UsingHubble's law, the redshift can be used to estimate the distance of an object fromEarth. By combining redshift with angular position data, a redshift survey maps the 3D distribution of matter within a field of the sky. These observations are used to measure detailed statistical properties of thelarge-scale structure of the universe. In conjunction with observations of early structure in thecosmic microwave background, these results can place strong constraints on cosmological parameters such as the average matter density[1][2] and the Hubble constant.[3]
Generally the construction of a redshift survey involves two phases: first the selected area of the sky is imaged with a wide-field telescope, then galaxies brighter than a defined limit are selected from the resulting images as non-pointlike objects; optionally, colour selection may also be used to assist discrimination between stars and galaxies.[4] Secondly, the selected galaxies are observed byspectroscopy, most commonly at visible wavelengths, to measure the wavelengths of prominentspectral lines; comparing observed and laboratory wavelengths then gives theredshift for each galaxy.
TheGreat Wall, a vast conglomeration of galaxies over 500 million light-years wide, provides a dramatic example of a large-scale structure that redshift surveys can detect.
The first systematic redshift survey was theCfA Redshift Survey of around 2,200 galaxies, started in 1977 with the initial data collection completed in 1982. This was later extended to the CfA2 redshift survey of 15,000 galaxies,[5] completed in the early 1990s.
These early redshift surveys were limited in size by taking a spectrum for one galaxy at a time; from the 1990s, the development of fibre-optic spectrographs and multi-slit spectrographs enabled spectra for several hundred galaxies to be observed simultaneously, and much larger redshift surveys became feasible. Notable examples are the2dF Galaxy Redshift Survey (221,000 redshifts, completed 2002); theSloan Digital Sky Survey (approximately 1 million redshifts by 2007) and theGalaxy And Mass Assembly survey. At high redshift the largest current surveys are theDEEP2 Redshift Survey and theVIMOS-VLT Deep Survey (VVDS); these have around 50,000 redshifts each, and are mainly focused on galaxy evolution.
ZFOURGE or the FourStar Galaxy Evolution Survey is a large and deep medium-band imaging survey which aims to establish an observational benchmark ofgalaxy properties atredshift z > 1. The survey is usingnear-infrared FOURSTAR instrument on theMagellan Telescopes, surveying in all three HST legacy fields: COSMOS, CDFS, and UDS.[6]
Because of the demands on observing time required to obtain spectroscopic redshifts (i.e., redshifts determined directly fromspectral features measured at high precision), a common alternative is to usephotometric redshifts based on model fits to the brightnesses and colors of objects. Such "photo-z's" can be used in large surveys to estimate the spatial distribution of galaxies and quasars, provided thegalaxy types andcolors are well understood in a particular redshift range. At present, the errors on photometric redshift measurements are significantly higher than those ofspectroscopic redshifts, but future surveys (for example, theLSST) aim to significantly refine the technique.