Earth-sized virtual telescope to study supermassive black hole at center of Milky Way

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The University of Arizona is helping to build a virtual radio telescope the size of the Earth with a resolution a thousand times greater than Hubble (Photo: ESO/B. Tafreshi/TWAN/twanight.org)

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In astronomy, much like many other other aspects of life, bigger is better. Taking this adage to heart, astronomers at the University of Arizona are helping to build a virtual radio telescope the size of the Earth itself. With a resolution factor more than a thousand times greater than that of the Hubble Space Telescope, the new Event Horizon Telescope (EHT) will be used to study in fine detail the supermassive black hole at the center of our Milky Way.

A team led by Dan Marrone, assistant professor of Astronomy and Steward Observatory at the University of Arizona, has recently helped stitch in the latest in a range of radio telescopes across the world that form part of a set of instruments being used for Very Long Baseline Interferometry, or "VLBI" – a technique whereby a telescope with a size equal to the maximum separation between multiple linked radio telescopes can be emulated. This latest instrument is located in Antarctica and is known as the South Pole Telescope (SPT). The connection of the SPT completes the loop of a range of other high-end observation devices spanning the globe.

"Now that we’ve done VLBI with the SPT, the Event Horizon Telescope really does span the whole Earth, from the Submillimeter Telescope on Mount Graham in Arizona, to California, Hawaii, Chile, Mexico, Spain and the South Pole," said Dan Marrone. "The baselines to SPT give us two to three times more resolution than our past arrays, which is absolutely crucial to the goals of the EHT. To verify the existence of an event horizon, the 'edge' of a black hole, and more generally to test Einstein's theory of general relativity, we need a very detailed picture of a black hole. With the full EHT, we should be able to do this."

Identified as being in the vicinity of Sagittarius A* (pronounced "A-star"), the supermassive black hole to be studied by the EHT is 26,000 light years away at the center of our galaxy. At a mass four million times greater than that of our own sun, and possessing a diameter equivalent to the width of the orbit of Mercury, Sagittarius A* certainly deserves its supermassive black hole title. However, at such great distances even this gargantuan object is difficult to see even with the most powerful telescopes.

This is where the EHT will take advantage of its Earth-sized capture area. With the feat of looking at the event horizon of this supermassive object being, as the team states, as problematic as trying to read the issue date on a penny in New York from somewhere in California, the resolution capabilities of this multi-faceted instrument cannot be understated. Incorporating an array of super-sensitive instruments from around the world, such as the ALMA in Chile, the enormous baseline from the sheer spread of telescopes criss-crossing the globe is hoped to achieve such resolution.

The latest addition to the EHT, and operating at millimeter radio wavelengths, the 10-meter (33-ft) diameter SPT component is used primarily to capture detailed images of background radiation in the cosmos – cosmic microwave radiation remnants of the Big Bang. Located all the way down in the Antarctic with clear skies free of water vapor and at a height of around 2,800 m (9,300 ft), the 23-meter (75-ft) tall, 34,000-kg (380-ton) SPT has, until now, been employed in the pursuit of fundamental answers around such cosmological questions as to what happened moments after the Big Bang, the nature of dark energy, and the process of inflation of our universe.

"We are thrilled that the SPT is part of the EHT," said John Carlstrom, professor in Astronomy and Astrophysics at the University of Chicago, who leads the SPT collaboration. "The science, which addresses fundamental questions of space and time, is as exciting to us as peering back to the beginning of the universe."

"…the supermassive black hole at the Milky Way’s center is always visible from the South Pole, so adding that station to the EHT is a major leap toward bringing an event horizon into focus." Added Shep Doeleman, assistant director at MIT’s Haystack Observatory.

Assimilating this latest link in the VLBI EHT chain involved a complex process of aligning all of the far-flung observatories on a single spot. This was achieved using a specially-constructed, single-pixel detector apparatus created by Marrone’s team. Using this device – in tandem with an atomic clock to precisely time the observations of incoming light – allowed all of the connected observatories worldwide to effectively amalgamate and act as one single observing device.

Now, with the SPT online, the observed microwave data from the array almost exceeds 200 terabytes every single day that it is in use. To capture this incredible amount of data, the team called on the Smithsonian Astrophysical Observatory and Haystack Observatory at MIT to produce equipment to capture and record all of this information. In effect, the MIT team was able to increase the capture rates four-fold over previous systems.

"To extend the EHT to the South Pole required improving our data capture systems to record data much more quickly than ever before," said Laura Vertatschitsch of the Smithsonian Astrophysical Observatory.

This observations should also shed light on the rapid motion of the stellar object G2 near Sagittarius A*, which – along with the study of other nearby stellar objects – has been used to postulate the existence of the supermassive black hole purported to be in that region. Studies using a range of methods over a period of almost two decades have all but confirmed this, but training the most sensitive radio telescope on the region where the event horizon of this object seems to exist should remove all doubt.

"VLBI is very technically challenging, and a whole system of components had to work perfectly at both SPT and APEX for us to detect our targets," said Junhan Kim, a doctoral student at the UA who helped build and install the SPT EHT receiver. "Now that we know how to incorporate SPT, I cannot wait to see what we can learn from a telescope 10,000 miles across."

In upcoming events, the new Earth-wide VLBI and its newly-incorporated SPT will be involved in their first look at the supermassive black hole in Sagittarius A*. Following this, even more instruments from other parts of the world will be prepared to come on line. Whilst this will not increase the Earth-wide coverage now achieved by the latest link to the Antarctic instrument, it will add further refinement and data-gathering capabilities that should provide an even more definitive look at the nature of the previously unobservable objects in our universe.

The video below provides background information on the EHT.

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