Editing Redshift survey (section)

[[Image:2dfgrs.png|right|thumb|300px|Rendering of the 2dFGRS data.]]
[[File:A slice through the Universe.webm|thumb|The positions in space of the galaxies identified by the VIPERS survey.]]

In [[astronomy]], a '''redshift survey''' is a [[astronomical surveys|survey]] of a section of the sky to measure the [[redshift]] of [[astronomical object]]s: usually galaxies, but sometimes other objects such as galaxy clusters or quasars.  
Using [[Hubble's law]], the redshift can be used to estimate the distance of an object from [[Earth]].  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 the [[Large-scale structure of the cosmos|large-scale structure]] of the universe. In conjunction with observations of early structure in the [[cosmic microwave background]], these results can place strong constraints on cosmological parameters such as the average matter density<ref>{{cite journal|last1=Peacock|first1=John A.|last2=Cole|first2=Shaun|last3=Norberg|first3=Peder|last4=Baugh|first4=Carlton M.|last5=Bland-Hawthorn|first5=Joss|last6=Bridges|first6=Terry|last7=Cannon|first7=Russell D.|last8=Colless|first8=Matthew|last9=Collins|first9=Chris|last10=Couch|first10=Warrick|last11=Dalton|first11=Gavin|last12=Deeley|first12=Kathryn
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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.<ref>{{cite web|title=SkyServer - Algorithms|url=http://skyserver.sdss.org/dr1/en/help/docs/algorithm.asp?key=target|website=sdss.org|accessdate=18 August 2014}}</ref> Secondly, the selected galaxies are observed by [[spectroscopy]], most commonly at visible wavelengths, to measure the wavelengths of prominent [[spectral line]]s; comparing observed and laboratory wavelengths then gives the [[redshift]] for each galaxy.

The [[Great Wall (astronomy)|Great 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 the [[CfA 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,<ref>{{cite journal|last1=Falco|first1=Emilio E.|last2=Kurtz|first2=Michael J.|last3=Geller|first3=Margaret J.|last4=Huchra|first4=John P.|last5=Peters|first5=James|last6=Berlind|first6=Perry|last7=Mink|first7=Douglas J.|last8=Tokarz|first8=Susan P.|last9=Elwell|first9=Barbara|title=The Updated Zwicky Catalog (UZC)|journal=Publications of the Astronomical Society of the Pacific|volume=111|issue=758|pages=438–452|doi=10.1086/316343|year=1999|arxiv = astro-ph/9904265 |bibcode = 1999PASP..111..438F |s2cid=14298026}}</ref> 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 the [[2dF Galaxy Redshift Survey]] (221,000 redshifts, completed 2002); the [[Sloan Digital Sky Survey]] (approximately 1 million redshifts by 2007) and the [[Galaxy And Mass Assembly survey]].  At high redshift the largest current surveys are the [[DEEP2 Redshift Survey]] and the [[VIMOS-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 of [[galaxy]] properties at [[redshift]] z > 1. The survey is using [[near-infrared spectroscopy|near-infrared]] FOURSTAR instrument on the [[Magellan Telescopes]], surveying in all three HST legacy fields: COSMOS, CDFS, and UDS.<ref>{{Cite web | url=http://zfourge.tamu.edu/ | title=Zfourge}}</ref>

Because of the demands on observing time required to obtain spectroscopic redshifts (i.e., redshifts determined directly from [[Spectral line|spectral features]] measured at high precision), a common alternative is to use [[photometric redshift]]s 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 the [[Galaxy#Types and morphology|galaxy types]] and [[Galaxy color-magnitude diagram|colors]] are well understood in a particular redshift range. At present, the errors on photometric redshift measurements are significantly higher than those of [[Spectroscopic astronomy|spectroscopic]] redshifts, but future surveys (for example, the [[Large Synoptic Survey Telescope|LSST]]) aim to significantly refine the technique.