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10 May 2010
The Karoo Array Telescope (MeerKAT) project, a world-class radio telescope being built by the Department of Science and Technology and the National Research Foundation in the Karoo semi-desert in South Africa near the towns of Carnarvon and Williston, achieved a major milestone in April 2010 when the first four telescopes in the MeerKAT Precursor Array (MPA - also known as KAT-7) were linked together as an integrated system to produce the MeerKAT's first interferometric image of an astronomical object.
Rapid progress has been made with the MeerKAT Precursor Array with the recent installation of receiver systems on four of the seven dishes now standing at the radio-quiet Square Kilometer Array (SKA) candidate site in the radio astronomy reserve in the Northern Cape province in South Africa. In December 2009, the first combined signals (or "fringes") were obtained from a range of celestial radio sources, using the first two fully equipped antennas. Then the first raster scan image was produced in March 2010 by scanning individual antennas across the Centaurus A radio galaxy (see figure 1, below). This provided a very successful end-to-end demonstration of antenna hardware, system stability, pointing accuracy and software. Once the antennas were demonstrated to work well individually, another major milestone was achieved: phase closure (using three antennas) and amplitude closure (using four antennas) which demonstrated that the underlying system was working as expected and the antennas could be linked together.
Now, a major new milestone has been achieved with the linking of four 12-meter diameter antennas in the MeerKAT Precursor Array (Figure 2) as they observed the Centaurus A galaxy simultaneously. The signals collected by the radio antennas were processed by state-of-the-art electronics and control software to transform the four antennas into a single, high-resolution telescope system, called an interferometer. The data were recorded to a computer disk and processed with advanced software to produce an image of a jet being ejected from the black hole the core of this amazing galaxy (see figure 1, below).
Centaurus A is a peculiar galaxy shrouded by a lane of gas and dust, created when a larger elliptical galaxy began to collide with a smaller spiral galaxy (see figure 1, below). This colliding galaxy is located about 14 million light-years away from earth in the constellation of Centaurus. Although it takes 14 million years for the light from Centaurus A to reach us, it is still one of the closest radio galaxies to earth. Radio galaxies are galaxies that emit large amounts of radio energy that is invisible to the naked eye and optical telescopes. Previous observations of Centaurus A by other radio telescopes and X ray telescopes have shown that a super-massive black hole at the center of the galaxy ejects a stream of high energy particles in a jet that is moving at about one half the speed of light (about 150,000,000 meters per second). As this jet flows into inter-galactic space, ultra-relativistic, charged particles in the million-degree gas move through magnetic fields producing radio emission (this process is called "synchrotron emission").
The spectacular Centaurus A galaxy is the fifth brightest in the sky and is well studied by amateur and professional astronomers alike. The galaxy is beautifully located high in the southern hemisphere sky near the well-known constellation of the Southern Cross. Because it is so well-studied and it is bright at radio wavelengths, Centaurus A is an ideal celestial object to verify that the complicated electronics and control software in the MeerKAT Precursor Array are working as expected and they produce images with structure that is consistent with what other telescopes have detected. This "commissioning" phase of the project is a vital test to prove that the underlying hardware and software, created by the expert team in South Africa and their industry partners, are ready for the next phase of construction.
The interferometric image of Centaurus A was produced from a 6-hour observation using non-cryogenically cooled, test receivers. Although the receivers were not cooled, the central core of the Centaurus A galaxy has been easily detected. The image was produced by the MeerKAT engineering team based in Cape Town and used software they developed
for the purpose of initial exploration and commissioning of the MeerKAT Pre-Cursor. No editing of the data was performed in the production of the first image, which shows the excellent radio quietness characteristics of the South African site. The data have been roughly calibrated and the final image has a signal-to-noise level of about 20. The image resolution is about 7'x4' with a position angle of about 6deg.
The image produced compares very favourably with what is expected. "We are extremely pleased by the rapid progress and early results obtained from the first MPA antennas", reports SKA SA Project Director Dr Bernie Fanaroff. "This shows that we are well on track with the development of our SKA site, and the development of the systems and understanding required to build and operate advanced scientific instruments such as MeerKAT and the SKA. It also shows that our team is as good as any in the world today and we are very proud of them."
This initial image of Centaurus A, although an important milestone, is a first demonstration only. A better image of this galaxy and other celestial sources will be possible once all seven antennas in the MeerKAT Precursor Array are brought together to form a fully-functional radio telescope array. However, the more well distributed antennas in an array, the better the quality of the image. That is why it is so important to build the MeerKAT array, with 80 antennas planned for construction. MeerKAT with its larger number of dishes and carefully optimized layout will make it possible to construct much higher quality radio images.
What will Centaurus A look like at radio wavelengths with the MeerKAT array? We expect it to look similar to the image shown in figure 3 - created by overlaying the optical image with an image of the radio emission mapped with the Very Large Array (VLA) located in New Mexico in the United States. After the initial verification process of MeerKAT is complete, a whole new world of discovery opens up as MeerKAT looks deep into the radio skies of the southern hemisphere with such power that it can even detect the earliest galaxies, born at the dawn of time.
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Figure 1: The many faces of Centaurus A, a giant galaxy located 14 million light years from earth.
The left panel shows a false-color image of the Centaurus A galaxy created by one antenna in the Karoo Array Telescope (MeerKAT) Precursor Array. The huge elongated structure is caused by a jet of high-speed gas being ejected from the core of the galaxy. The image measures 9 degrees from top to bottom - about the size of 18 full moons placed end to end! The heart of the jet emanating from the galaxy is seen in the top right panel: the first image created by combining four MeerKAT Precursor Array antennas together to form a single, high-resolution telescope system. At the very center of this jet is the Centaurus A galaxy as seen in optical light in the lower right panel (this image was created by the NASA Space Telescope Science Institute and the European Space Agency Information Centre). Picture by Sharmila
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Figure 2: The Karoo Array Telescope (MeerKAT) Precursor Array.
The MeerKAT Precursor Array showing six of the seven antennas is seen against the splendor of the setting sun in the Karoo. The antenna on the far left was used to create the "raster" image of Centaurus A, showing the large-scale emission from the jet in the galaxy. Four of the antennas outfitted with receivers to detect radio waves were used to create an "interferometric" image of the jet coming from the heart of Centaurus A.
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Figure 3: What MeerKAT will be able to image.
When MeerKAT is built, the power of 80 antennas, combined to operate as a single, high-resolution telescope, will display the awesome beauty of the universe. Like this image of Centaurus A, created with the NRAO's Very Large Array radio telescope and NASA's Hubble Space Telescopes, MeerKAT will demonstrate just how amazing the universe really is. Images and discoveries from MeerKAT will inspire our young and bring South Africa into the forefront of astronomical research in the world.
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Lillian Mofokeng
Communications Manager
Square Kilometre Array South African Project
Tel: +27 (0) 11 442 2434
Cell:+27 (0) 82 888 8362
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