Astronomers building an Earth-sized virtual telescope capable of photographing the event horizon of the black hole at the center of our Milky Way have extended their instrument to include the University of Chicago-built South Pole Telescope.
The South Pole Telescope, situated at the National Science Foundation’s Amendsen-Scott South Pole Station, now is part of the largest virtual telescope ever built—the Event Horizon Telescope. By combining telescopes across the Earth, the Event Horizon Telescope will take the first detailed pictures of black holes.
“We are thrilled that the South Pole Telescope is part of the EHT. The science, which addresses fundamental questions of space and time, is as exciting to us as peering back to the beginning of the universe,” said UChicago Prof. John Carlstrom, who leads the SPT collaboration.
The Event Horizon Telescope is an array of radio telescopes connected using a technique known as Very Long Baseline Interferometry. Larger telescopes can make sharper observations, and interferometry allows multiple telescopes to act like a single telescope as large as the separation, or “baseline,” between them.
Now that the technique has been extended to the South Pole Telescope, the Event Horizon Telescope spans the entire Earth, from the Submillimeter Telescope on Mount Graham in Arizona, to California, Hawaii, Chile, Mexico, Spain and the South Pole.
“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,” said the University of Arizona’s Dan Marrone, who leads the effort. “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.”
The prime Event Horizon target is the Milky Way’s black hole, known as Sagittarius A* (pronounced ‘A-star’). Even though it is four million times more massive than the sun, it is tiny to the eyes of astronomers. Smaller than Mercury’s orbit around the sun, yet almost 26,000 light years away, studying its event horizon in detail is equivalent to standing in California and reading the date on a penny in New York.
With its unprecedented resolution, more than 1,000 times better than the Hubble Space Telescope, the Event Horizon Telescope will see swirling gas on its final plunge over the event horizon—never to regain contact with the rest of the universe. If the theory of general relativity is correct, the black hole itself will be invisible because not even light can escape its immense gravity.
First postulated by Albert Einstein’s General Theory of Relativity, the existence of black holes has since been supported by decades’ worth of astronomical observations. Most, if not all galaxies, are now believed to harbor a supermassive black hole at their center, and smaller ones formed from dying stars should be scattered among their stars. The Milky Way is known to be home to about 25 smallish black holes, ranging from five to 10 times the sun’s mass. But never has it been possible to directly observe and image one of these cosmic oddities.
Weighing 280 tons and standing 75 feet tall, the South Pole Telescope sits at an elevation of 9,300 feet on the polar plateau at Amundsen-Scott, which is located at the geographic South Pole. The telescope was built and is supported by the National Science Foundation’s Division of Polar Programs. The division manages the U.S. Antarctic Program, which coordinates all U.S. research on the southernmost continent.
The 10-meter South Pole Telescope operates at millimeter wavelengths to make high-resolution images of cosmic microwave background radiation, the light left over from the big bang. Because of its location at the earth’s axis and at high elevation where the polar air is largely free of water vapor, it can conduct long-term observations to explore some of the biggest questions in cosmology, such as the nature of dark energy and the process of inflation that is believed to have stretched the universe exponentially in a tiny fraction of the first second after the big bang.
To incorporate South Pole Telescope into the Event Horizon Telescope, Marrone’s team constructed a special, single-pixel camera that can sense the microwaves hitting the telescope. The Academia Sinica Institute for Astronomy and Astrophysics in Taiwan provided the atomic clock needed to precisely track the arrival time of the light. Comparing recordings made at telescopes all over the world allows the astronomers to synthesize the immense telescope. The Smithsonian Astrophysical Observatory and MIT Haystack Observatory provided equipment to record the microwaves at incredibly high speeds, generating nearly 200 terabytes per day.
“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. A new “digital back end,” developed by Vertatschitsch and colleagues, can process data four times faster than its predecessor, which doubles the sensitivity of each telescope.
For their preliminary observations, Marrone and his associates trained their instrument on two known black holes, Sagittarius A* in our galaxy, and another, located 10 million light years away in a galaxy named Centaurus A. For this experiment, the South Pole and the APEX telescope in Chile observed together, despite being nearly 5,000 miles apart. These data constitute the highest resolution observations ever made of Centaurus A (though the information from a single pair of telescopes cannot easily be converted to a picture).
The next step will be to include the South Pole Telescope in the annual Event Horizon experiments that combine telescopes all over the world. Several new telescopes have been preparing to join the Event Horizon Telescope in the next year, meaning that the next experiment will be the largest both geographically and with regard to the number of telescopes involved.
Funding: NSF grants AST-1207752 to Dan Marrone; AST-1207704 to Sheperd Doeleman at MIT’s Haystack Observatory; and AST-1207730 to John Carlstrom at the University of Chicago.