astronomy Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...

, a redshift survey is a

survey Survey may refer to: Statistics and human research * Statistical survey, a method for collecting quantitative information about items in a population * Survey (human research), including opinion polls Spatial measurement * Surveying, the techniq ...

of a section of the sky to measure the redshift of astronomical objects: usually galaxies, but sometimes other objects such as galaxy clusters or quasars. Using

Hubble's law Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving a ...

, the redshift can be used to estimate the distance of an object from

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

. 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 universe. In conjunction with observations of early structure in the

cosmic microwave background In Big Bang cosmology the cosmic microwave background (CMB, CMBR) is electromagnetic radiation that is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all spac ...

, these results can place strong constraints on cosmological parameters such as the average matter density and the Hubble constant. 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. Secondly, the selected galaxies are observed by spectroscopy, most commonly at visible wavelengths, to measure the wavelengths of prominent

spectral line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...

s; comparing observed and laboratory wavelengths then gives the redshift for each galaxy. The

Great Wall The Great Wall of China (, literally "ten thousand ''li'' wall") is a series of fortifications that were built across the historical northern borders of ancient Chinese states and Imperial China as protection against various nomadic groups ...

, 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, 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 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 a very efficient near-infrared FOURSTAR i ...

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 Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...

FOURSTAR instrument on the

Magellan Telescopes The Magellan Telescopes are a pair of optical telescopes located at Las Campanas Observatory in Chile. The two telescopes are named after the astronomer Walter Baade and the philanthropist Landon T. Clay. First light for the telescopes was on ...

, surveying in all three HST legacy fields: COSMOS, CDFS, and UDS. Because of the demands on observing time required to obtain spectroscopic redshifts (i.e., redshifts determined directly from spectral features measured at high precision), a common alternative is to use

photometric redshift A photometric redshift is an estimate for the recession velocity of an astronomical object such as a galaxy or quasar, made without measuring its spectrum. The technique uses photometry (that is, the brightness of the object viewed through various ...

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

colors Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are associa ...

are well understood in a particular redshift range. At present, the errors on photometric redshift measurements are significantly higher than those of

spectroscopic Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...

redshifts, but future surveys (for example, the LSST) aim to significantly refine the technique.

References

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External links

Probes of Large Scale Structure

Physical cosmology Observational astronomy Large-scale structure of the cosmos

See also

References

External links