Sunyaev–Zel'dovich Array
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The Sunyaev–Zeldovich Array (SZA) in California is an array of eight 3.5 meter telescopes that was operated as part of the now-closed
Combined Array for Research in Millimeter-wave Astronomy The Combined Array for Research in Millimeter-wave Astronomy (CARMA) was an astronomical instrument comprising 23 radio telescopes, dedicated in 2006. These telescopes formed an astronomical interferometer where all the signals are combined in a ...
(CARMA). Its initial goals were to survey the
cosmic microwave background The cosmic microwave background (CMB, CMBR), or relic radiation, is microwave radiation that fills all space in the observable universe. With a standard optical telescope, the background space between stars and galaxies is almost completely dar ...
(CMB) in order to measure its fine-scale
anisotropies Anisotropy () is the structural property of non-uniformity in different directions, as opposed to isotropy. An anisotropic object or pattern has properties that differ according to direction of measurement. For example, many materials exhibit ver ...
and to find clusters of galaxies. The survey was completed in 2007, and the array is now used primarily to characterize clusters via the
Sunyaev–Zeldovich effect The Sunyaev–Zeldovich effect (named after Rashid Sunyaev and Yakov B. Zeldovich and often abbreviated as the SZ effect) is the spectral distortion of the cosmic microwave background (CMB) through inverse Compton scattering by high-energy e ...
. Observations commenced at the SZA in April 2005. One of the most important developments of the last few years has been the detection, through observations of the CMB and
supernova A supernova (: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last stellar evolution, evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion ...
studies, of a form of energy that is accelerating the expansion of the universe. Dubbed
dark energy In physical cosmology and astronomy, dark energy is a proposed form of energy that affects the universe on the largest scales. Its primary effect is to drive the accelerating expansion of the universe. It also slows the rate of structure format ...
by analogy with dark matter, it is believed to account for roughly 70% of the universe's energy content. While dark energy cannot be observed directly, its basic properties can be inferred from its effect on structure formation in the universe. Just as an ecologist can learn about the food supply by studying how animal populations evolve with time, physicists can learn about dark energy by studying the population statistics of the universe's inhabitants—in this case,
galaxy clusters A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity, with typical masses ranging from 1014 to 1015 solar masses. Clusters consist of galax ...
. The SZA gets its name from the means by which it measures galaxy clusters: the scattering of CMB light as it passes through the hot ionized cluster gas, known as the Sunyaev–Zeldovich effect (SZ effect). In short, the CMB is used as a backlight against which galaxy clusters can be seen by the shadows they cast. Since the SZA sees the shadow rather than the light emitted by the cluster itself, it can be used to measure sufficiently large clusters nearly independently of their
redshift In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and increase in frequency and e ...
, back to the epoch at which clusters first began to form.


Experiment

The SZA has been used for multi-wavelength observations of over 100 galaxy clusters, both on its own and as a part of the
Combined Array for Research in Millimeter-wave Astronomy The Combined Array for Research in Millimeter-wave Astronomy (CARMA) was an astronomical instrument comprising 23 radio telescopes, dedicated in 2006. These telescopes formed an astronomical interferometer where all the signals are combined in a ...
(CARMA), which was decommissioned after 3 April 2015. From 2005 to 2007, SZA undertook a deep 31 GHz (
Gigahertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose formal expression in terms of SI base un ...
) survey of several patches of sky.


Instrument

The SZA is not a single telescope, but an array of 8 telescopes operating together as an
interferometer Interferometry is a technique which uses the '' interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber opt ...
. An interferometer does not detect light in quite the same way as an ordinary telescope, by measuring the total power collected by a single dish; instead, it looks at differences between the light falling on pairs of telescopes. Like water waves, light waves can interfere with each other, producing a complex pattern of intensity enhancements where the waves constructively interfere, and nulls where they destructively interfere. As light from a source washes over the array, an interferometer detects this
interference pattern In physics, interference is a phenomenon in which two coherent waves are combined by adding their intensities or displacements with due consideration for their phase difference. The resultant wave may have greater amplitude (constructive int ...
— hence the name. The source's structure on the sky can then be inferred from the interference pattern in much the same way that one might infer the size and shape of a stone thrown into a pond from the pattern of ripples left in its wake. The native resolution of an interferometer depends not on the size of the individual telescopes (as with a traditional single telescope), but on their separation. Pairs of telescopes with large separations provide sensitivity to small-scale structure, while short spacings are sensitive to large-scale structure on the sky. The 8 SZA telescopes are small enough to be placed very close together, which provides maximum sensitivity to the (large-scale) SZ signal from clusters. When the SZA was combined with the other telescopes in the CARMA array, which had longer separations and were sensitive to finer angular scales, it formed a complete picture of galaxy clusters at very high resolution.


References

{{DEFAULTSORT:Sunyaev-Zeldovich Array Radio telescopes Interferometric telescopes Cosmic microwave background experiments Buildings and structures in Inyo County, California