Square kilometre array
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Author: Dr. Rainer Beck, Max-Planck-Institut für Radioastronomie, Bonn, Germany
Square Kilometre Array
Understanding the evolution of the Universe, galaxies and stars requires looking back in time as far as possible. But the radiation from distant objects is incredibly weak and its detection needs huge collecting areas. Increasing sensitivity will reveal new classes of cosmic objects, distant and nearby, which are too faint or too short-lived to be detected so far. Radio waves carry signals from gas clouds emitted even before the formation of the first stars. Until the next decade a new telescope will become the largest telescope ever built: the Square Kilometre Array (SKA) [1].
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Radio waves
Radio waves provide a number of advantages: they do not suffer from distortions in the atmosphere (scintillation), they are not absorbed by interstellar dust like optical waves, and they emerge from objects widely different from the well-known sources of light. Some of the most spectacular objects in the Universe are radio sources whose radiation is emitted from hot gas and charged particles around black holes (quasars) and in the magnetospheres around neutron stars (pulsars). Cold gas in galaxies, invisible in the optical range, can be radio-bright when emitting in specific radio spectral lines. Radio waves tell us that the Universe does not only consist of stars, gas and dark matter, but is also permeated by superfast “cosmic ray” particles and magnetic fields which emit synchrotron emission over a wide (continuous) frequency range in the radio, while they escape detection in most other spectral ranges. Radio astronomy is another window to the Universe where known objects look different and new objects shine. The radio window allows us to look deep into space and hence deep into the past, and we can observe how the gas, fast particles and magnetic fields developed over the times. Scientists worldwide are excited about the possibilities offered by the SKA (see below).
Technical design
With a collecting area of about one square kilometer, the SKA will be about ten times more sensitive than the largest single dish telescope at Arecibo (Puerto Rico) [2], and fifty times more sensitive than the currently most powerful interferometer, the Expanded Very Large Array (EVLA, at Socorro/USA) [3]. The SKA will continuously cover most of the frequency range accessible from ground, from 70 MHz to 10 GHz (corresponding to wavelengths of 3 centimeters to 4 meters) in the first and second phases, later to be extended to at least 30 GHz (1 centimeter). The third major improvement is the enormously wide field of view, ranging from 200 square degrees at 70 MHz to at least 1 square degree at the highest frequency. The speed to survey a large part of the sky will hence be ten thousand to a million times faster than what is possible today.
To meet these ambitious specifications and the cost limit, planning and construction of the SKA is a major challenge in many technological areas, such as light low-cost antennas, detector arrays with a wide field of view, low-noise amplifiers, high-capacity data transfer, high-speed parallel-processing computers and high-capacity data storage units. The realization needs multifold innovative solutions which will soon find their way into general communication technology.
The frequency range spanning more than two decades cannot be realized with one single antenna design, so this will be achieved with a combination of different types of antennas. Under investigation are:
1. An aperture array of simple dipole antennas with wide spacings (a “sparse aperture array”) for the low-frequency range (from 70 MHz to about 300 MHz) (Fig. 1). This is a software telescope with no moving parts, steered solely by electronic phase delays, it has a large field of view and can observe towards several directions simultaneously. One station will contain 90 crossed dipole elements. 125 of these stations will be located within the 5 km diameter central core. Further 83 stations extend in the mid-region out to 180 km from the core.
2. An array of several thousand parabolic dishes of 12-15 meters diameter each for the medium frequency range (from about 300 MHz to 10 GHz), each equipped with a wide-bandwidth single-pixel feed (horn) (Fig. 2). Dishes will be built in the central core, in the 83 stations of the mid-region, and in smaller “remote” stations, each of 20 dish antennas, located along five huge spiral lines emerging from the core station out to distances of about 3000 km.
These two arrays constitute the baseline design for the SKA.
3. Two additional technologies for substantially enhancing the field of view in the 500-1000 MHz range are under rapid development: aperture arrays for medium frequencies with dense spacings (Fig. 3) and phased-array feeds for the parabolic dishes (see below). The technologies for a wide field of view are currently less mature than the dishes and the low-frequency dipole array but have the promise of significant scientific benefit in further increasing the survey speed if they prove feasible and cost effective.
Technical developments
Examples of low frequency arrays, as ”pathfinder” telescopes for the SKA, are the European LOFAR (Low Frequency Array) telescope, with its core in the Netherlands [4], the Murchison Widefield Array, MWA, in Australia [5], the Long Wavelength Array, LWA, in the USA [6], and the Precision Array to Probe the “Epoch of Reionization” (see below), PAPER, also in Australia [7]. All these long wavelength telescopes are software telescopes steered by electronic phase delays (“phased aperture array”). The first LOFAR stations saw “first light” in 2007 in the frequency band 30-80 MHz and in 2009 in the frequency band 110-240 MHz (Fig. 4). Full operation of LOFAR is expected in late 2010.
Examples of dishes with a single-pixel feed are operating already in the USA (Allen Telescope Array, ATA) [8] and are under development in South Africa (MeerKAT) [9]. The first 12-meter prototype dish of the MeerKAT array, one of the SKA precursor telescopes, has been completed in 2009.
Dense aperture arrays comprise up to millions of receiving elements in planar arrays on the ground which can be phased together to point in any direction on the sky. Due to the large reception pattern of the basic elements, the field of view can be up to 250 square degrees. Dense aperture arrays have been the subject of a European Commission-funded design study named SKA Design Study (SKADS) which has resulted in a prototype array of 140 square meters area (EMBRACE) [10].
The technology of dense phased-array feeds can also be adopted to the focal plane of parabolic dishes. Such a “radio camera” is composed of many elements (“pixels”) which are controlled and combined electronically. This allows the dishes to observe over a far wider field of view than when using a classical single-pixel feed horn. Prototypes of such wide-field cameras are under construction in the Netherlands (APERTIF) [11], Australia (ASKAP) [12], and in Canada (PHAD) [13]. The construction of the first 36 dish antennas (12m size) of ASKAP, the other precursor telescope for the SKA, will start in Western Australia in 2010.
Computing requirements
The individual antennas will be grouped into the large core area of 5 km diameter, with separate cores for the dish antennas and the two types of aperture arrays (Fig. 5), the mid-region with 83 stations with dish antennas and the phased arrays which are randomly spaced out to about 180 km, and smaller “remote” stations out to distances of about 3000 km with dish antennas only. The overall size determines the angular resolution which will be about 0.1 seconds of arc at 100 MHz and 0.001 seconds of arc at 10 GHz.
To obtain radio images with such resolutions, the data from all stations have to be transmitted to a central computer and processed online. Compared to LOFAR with a data rate of about 150 Gigabits per second and a central processing power of 27 Tflops, the SKA will produce more data and need much more processing power - by a factor of at least one hundred. Following “Moore’s law” of increasing computing power, a processor with sufficient power should be available by late next decade. The energy consumption for computers and cooling will be enormous.
Timeline
The final technical design will be selected in 2012 with construction planned to start in 2015. In the first phase (until 2018), about 10% of the SKA will be erected, with completion of construction at the low and mid frequency bands by about 2022. Two candidate sites fulfilling the scientific and logistical requirements have been short-listed: Australia, with the core in Western Australia and extending across to New Zealand, or southern Africa, with the core in South Africa and extensions to Madagascar and Mauritius. The site selection will be made at government level and is also expected in 2012. The total costs of the SKA are about 1.5 billion € (estimate from 2007), to be shared among the countries of the worldwide collaboration.
Key Science Projects
Such a huge investment needs convincing justification. Apart from the expected technological spin-offs, five main science questions (“key science”) drive the construction of the SKA:
- Probing the dark ages
According to present-day cosmological models, the Universe became transparent about 380.000 years after the big bang (at a redshift of about 1100). The radiation released at that time is now prominent in the radio range as the “Cosmic Microwave Background (CMB)”, measured in great detail by NASA’s WMAP satellite [14] and since 2009 by ESA’s PLANCK satellite [15]. Matter (mostly hydrogen) remained neutral and smoothly distributed over the next billion years, called the “dark ages”, until the first stars and black holes formed, followed by the formation of galaxies. The energy output from the first energetic stars and the jets launched near young black holes (quasars) started to heat the neutral gas, forming bubbles of ionized gas as structure emerged. The signatures from this exciting transition phase should still be observable with help of the radio line of hydrogen, though extremely redshifted by a factor of about 10 when arriving at our telescopes today (Fig. 6). The lowest SKA frequency will allow us to detect hydrogen at redshifts of up to 20, well into the “dark ages”, to search for the transition from a neutral to an ionized Universe, and hence provide a critical test of our present-day cosmological model.
- Galaxy evolution, cosmology, and dark energy
The expansion of the Universe is currently accelerating, a poorly understood phenomenon, for which a multitude of possible explanations have been proposed: Einstein's “cosmological constant” [16], a time-dependent energy called “quintessence”, topological defects, the effects of “other” Universes and many more. As long as the correct answer is not known, physicists and astronomers named the phenomenon “dark energy” [17].
One important method of distinguishing between these various explanations is to compare the distribution of galaxies at different epochs in the evolution of the Universe to the distribution of matter at the time when the Cosmic Microwave Background (CMB, see above) was formed, about 380.000 years after the Big Bang. Small distortions (“ripples”) in the distribution of matter, called “baryon acoustic oscillations”, should persist from the era of CMB formation until today. Tracking if and how these ripples change in size over cosmic age can then tell us if one of the existing models for dark energy is correct or a new idea is needed.
The SKA will use the hydrogen emission from galaxies to measure the properties of dark energy. Fortunately, the strongest line emission of hydrogen is in the radio range at a frequency of 1.4 GHz (21 cm wavelength), but redshifted to lower frequencies/longer wavelengths for distant galaxies. A deep all-sky SKA survey will detect hydrogen emission from galaxies out to redshifts of about 1.5, at a distance of about 9 billion light years, or at a time when the Universe was about 4.7 billion years old. The galaxy observations will be “sliced” in different redshift (time) intervals and hence reveal a comprehensive picture of the Universe's history.
As an important byproduct, we will learn how the hydrogen gas was concentrated to form galaxies and how fast it was transformed into stars. The hydrogen survey will simultaneously give us the synchrotron intensity of all galaxies which is a precise measure of their star-formation rate and magnetic field strength.
- Tests of general relativity and detection of gravitational waves with pulsars and black holes
Einstein’s Theory of General Relativity has precisely predicted the outcome of every test experiment so far. However, no tests in the strong gravitational field around black holes have yet been made. The SKA will search for a radio pulsar orbiting around a black hole (Fig. 7), the remnants from the supernova explosions of two massive stars in a binary system. Pulsars are precise clocks and will reveal time delays in extremely curved space with much higher precision than with laboratory experiments and hence probe the limits of General Relativity.
Regular high-precision observations of a network of pulsars with the SKA opens the way to detect gravitational waves with wavelengths of many light years in length, as expected for example from two massive black holes orbiting each other with a period of a few years resulting from galaxy mergers in the early Universe. When such a gravitational wave passes by the Earth, the space-time near is slightly changing at a frequency of a few nHz (about 1 oscillation per 30 years). The wave can be detected as apparent systematic delays and advances of the pulsar clocks in particular directions relative to the wave propagation in the sky.
We expect that more than 20.000 new pulsars will be detected with the SKA, compared to about 2000 known today. Almost all pulsars in the Milky Way (Fig. 8) and several 100 bright pulsars in nearby galaxies will become observable.
- Origin and evolution of cosmic magnetism
Electromagnetism is one of the fundamental forces, but little is known about its role in the Universe. Large-scale electric fields induce electric currents and are unstable, whereas magnetic fields can exist over long times because, mysteriously, single magnetic charges (monopoles) are missing in the Universe. It seems that all interstellar and probably intergalactic space is permeated by magnetic fields, but these are extremely hard to observe. Radio waves provide two tools: synchrotron radiation emitted by cosmic-ray electrons spiraling around magnetic field lines with almost the speed of light, and Faraday rotation of the polarization plane when a polarized (synchrotron) radio wave passes a medium with magnetic fields and thermal electrons. Both methods have been applied to reveal the large-scale magnetic fields in our Milky Way, nearby spiral galaxies (Fig. 9), and in galaxy clusters, which are probably amplified and maintained by dynamo action [18], but little is known about magnetic fields in the intergalactic medium. Furthermore, the origin and evolution of magnetic fields is still unknown. The first “seed” fields may originate in the very young Universe or may have been ejected from the first quasars, stars, or supernovae.
The SKA will measure the Faraday rotation towards several tens of million polarized background sources (mostly quasars), allowing us to derive the magnetic field structures and strengths of the intervening objects, such as, the Milky Way, distant spiral galaxies, clusters of galaxies, and in intergalactic space.
- The Cradle of life
The presence of life on other planets is a fundamental issue for astronomy and biology. The SKA will contribute to this question in several ways. Firstly, it will be able to detect the thermal radio emission from centimeter-sized “pebbles” in protoplanetary systems (Fig. 10) which are thought to be the first step in assembling Earth-like planets. The SKA will allow us to detect a protoplanet separated from the central star similar to the Sun-Earth separation out to distances of about 3000 light years.
More evolved Earth-like planets are observable via the line emissions of biomolecules in the radio range, as for example, “cold sugar” glycolaldehyde (CH2OHCHO) which has several lines between 13 and 22 GHz.
Finally, the SETI (Search for Extra Terrestrial Intelligence) [19] project will use the SKA to find hints of technological activities. Ionospheric radar experiments similar to those on Earth will be detectable out to several thousand light years, and Arecibo-type radar beams, like those that we use to map our neighbor planets in the solar system, out to as far as a few ten thousand light years. SETI will also search for such artificial signals superimposed onto natural signals from other projects.
Exploration of the Unknown
While the experiments described above are exciting science, the history of science tells us that many of the greatest discoveries happen unexpectedly and revealed objects which are completely different from those which had been envisaged during the planning phase of a new-generation telescope. For example, the serendipitous discovery of pulsars was made with a low-frequency telescope at Cambridge/UK that had been designed to measure the fluctuations of radio waves in interplanetary space. The unique sensitivity of the SKA will certainly reveal new classes of cosmic objects which are totally beyond our present imagination. We are looking forward to such surprises.
Further reading
The Square Kilometre Array, download from: http://www.skatelescope.org/PDF/brochure/SKABrochure_2008.pdf
C. Carilli and S. Rawlings: Science with the Square Kilometre Array, New Astronomy Reviews, vol. 48, Elsevier, Amsterdam (2004)
P. Hall: The SKA: an Engineering Perspective, Experimental Astronomy, vol. 17, Springer, Berlin (2005)
J. Lazio, M. Kramer and B. Gaensler: Tuning in to the Universe, Sky & Telescope 7/2008, p.20
Recommended reading
B.F. Burke, F. Graham-Smith: An Introduction to Radio Astronomy, 3rd ed., Cambridge University Press
T.L. Wilson, K. Rohlfs and S. Hüttemeister: Tools of Radio Astronomy, 5th ed., Springer Berlin 2009
