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Tip Sheet: March 30, 2017 at 8:13 pm

ALMA and the Event Horizon Telescope: Tip Sheet

Observations Scheduled to Begin April 5; Regular updates will be posted here

Topics in This Issue:
1. Imaging the Black Hole at the Center of Our Galaxy

On April 5, 2017, a team of astronomers, engineers, and technicians will attempt something unprecedented; they will link together a worldwide network of radio telescopes -- including the Atacama Large Millimeter/submillimeter Array (ALMA), with the goal of imaging the outer edges of a supermassive black hole.

2. What Is the Event Horizon Telescope?

The Event Horizon Telescope (EHT) is a worldwide network of radio astronomy facilities linked together with the goal of studying one of the most exciting objects in the known universe -- the edge of a black hole.

3. Black Holes and the Event Horizon

Supermassive black holes lurk at the center of all galaxies and contain millions or even billions of times the mass of our Sun.

4. Shadowy Science -- A Major Science Goal for the ALMA-enabled EHT

The light-bending power of black holes also presents a unique opportunity to observe the so-called “shadow” of a black hole.

5. Upgrading ALMA to be Part of the EHT

ALMA was designed to work as an interferometer -- but that type of telescope is incompatible with the EHT. Recently, astronomers upgraded ALMA so it could become a phased array.

1. Imaging the Black Hole at the Center of Our Galaxy

On April 5, 2017, a team of astronomers, engineers, and technicians will attempt something unprecedented; they will link together a worldwide network of radio telescopes — including the Atacama Large Millimeter/submillimeter Array (ALMA), with the goal of imaging the outer edges of a supermassive black hole.

Black holes are reality-bending concentrations of matter in space. They can be forged when star at least five times the mass of our Sun dies a spectacular death in a supernova explosion. The collapsing core from this explosion becomes so dense its gravity prevents even the fleeting particles of light from escaping its grasp.

Other black holes, millions to billions of times more massive than our Sun, reside at the centers of galaxies. These supermassive black holes exert tremendous influence on their home galaxies, especially when they gorge on gas and stars.

Though astronomers have long studied the impact of black holes on the universe, no one has ever imaged the actual point of no return, where matter and energy cannot escape a black hole — the so-called event horizon.

By combining the collecting area of ALMA and other millimeter-wavelength telescopes scattered across the globe, the EHT may finally achieve that goal.

To learn more about radio telescopes, arrays, and the key to the EHT — Very Long Baseline Interferometry — read here

Additional details are on the Joint ALMA Observatory website and the ESO website

The supermassive black hole at the center of our galaxy is hidden behind dense clouds of dust and gas. With the combined power of a worldwide network of radio telescopes, astronomers hope to peer into the heart of our galaxy and image -- for the first time -- the very edges of a black hole. When this network observes radio waves of one millimeter wavelengths, its magnifying power is high enough to see details at the black hole boundary.

Credit: NRAO/AUI/NSF

2. What Is the Event Horizon Telescope?

The Event Horizon Telescope (EHT) is a worldwide network of radio astronomy facilities linked together with the goal of studying one of the most exciting objects in the known universe — the edge of a black hole.

The EHT derives its extreme magnifying power by connecting widely spaced radio dishes across the globe into an Earth-sized virtual telescope. This technique, called Very Long Baseline Interferometry (VLBI), is the same process that enables telescopes like the Very Long Baseline Array (VLBA) and the Atacama Large Millimeter/submillimeter Array (ALMA) to achieve such amazing power and resolution. The difference between existing VLBI facilities and the EHT is the sheer geographical scope of the EHT project, its extension to the shortest observing wavelengths, and addition of the unprecedented collecting area enabled by ALMA.

The EHT will observe the center of our galaxy at a wavelength of 1.3 millimeters. This particular wavelength is essential to peer into the otherwise obscuring veil of dust and gas near the center of our galaxy. The telescope will achieve an astounding resolution of 10 – 20 microarcseconds — which is the equivalent of reading the date on a coin in Los Angeles from the distance of New York City.

The center of our galaxy, as seen with the VLA, Hubble, and Chandra.

Credit: NRAO/AUI/NSF; NASA Hubble, Chandra

3. Black Holes and the Event Horizon

Supermassive black holes lurk at the center of all galaxies and contain millions or even billions of times the mass of our Sun. These space-bending behemoths are so massive that nothing, not even light, can escape their gravitational influence. Understanding how a black hole devours matter, powers jets of particles and energy, and distorts space and time are leading challenges in astronomy and physics.

The black hole at the center of the Milky Way is a 4 million solar mass giant located approximately 26,000 light-years from Earth in the direction of the constellation Sagittarius. It is shrouded from optical telescopes by dense clouds of dust and gas, which is why observatories like ALMA, which operate at the longer millimeter and submillimeter wavelengths, are essential to study its properties.

Supermassive black holes can be relatively tranquil or they can flare up and drive incredibly powerful jets of subatomic particles deep into intergalactic space; quasars seen in the very early Universe are an extreme example. The fuel for these jets comes from in-falling material, which becomes superheated as it spirals inward. Astronomers hope to capture our Galaxy’s central black hole in the process of actively feeding to better understand how black holes affect the evolution of our Universe and how they shape the development of stars and galaxies.

High resolution imaging of the event horizon also could improve our understanding of how the highly ordered Universe as described by Einstein meshes with the messy and chaotic cosmos of quantum mechanics – two systems for describing the physical world that are woefully incompatible on the smallest of scales.

The view of the center of our galaxy with a closer view of the object known as Sagittarius A*, the bright radio source that corresponds to the supermassive black hole.

Credit: NRAO/AUI/NSF

4. Shadowy Science -- A Major Science Goal for the ALMA-enabled EHT

The light-bending power of black holes also presents a unique opportunity to observe the so-called “shadow” of a black hole. Light near the event horizon of a black hole does not travel in a straight line, but instead takes on weird hyperbolic trajectories and can even achieve a stable orbit. Some of this light, which begins its journey traveling away from observers on Earth, can get twisted back around, warping in such a way that it takes a 180 degree turn. This would allow scientists to study the far-side of a black hole and actually see its shadow in space. Since the size and shape of this shadow depends on the mass and spin of black hole, these observations could tell us much about how space and time are warped in this extreme environment.

Calculations indicate a resolution of 50 micro-arcseconds (approximately 2,000 times finer than the Hubble Space Telescope) is needed to image the shadow effect. That’s equivalent to reading the date on a quarter at the distance from New York to Los Angeles. This amazing high-resolution imaging is within the reach of the ALMA-enabled Event Horizon Telescope.

The center of the ALMA array on the Chajnantor Plateau at 5,000 meters.

Credit: NRAO/AUI/NSF

5. Upgrading ALMA to be Part of the EHT

ALMA was designed to work as an interferometer – a telescope made up of many individual elements. Each antenna pair creates a single baseline. ALMA can produce as many as 1,291 baselines, some up to 16 kilometers long.

But before ALMA could join the Event Horizon Telescope network, it first had to transform into a different kind of instrument known as a phased array. This new version of ALMA allows its 66 antennas to function as a single radio dish 85 meters in diameter. It’s this unified power coupled with ultraprecise timekeeping that allows ALMA to link with other observatories.

A major milestone along this path was achieved in 2014 when the science team performed what could be considered a “heart transplant” on the telescope by installing a custom-built atomic clock powered by a hydrogen maser. This new timepiece uses a process similar to a laser to amplify a single pure tone, cycles of which are counted to produce a highly accurate ‘tick’.

Shep Doeleman, the principal investigator of the ALMA Phasing Project, participated during the maser installation via remote video link. “ALMA will use the ultraprecise ticking of this new atomic clock to join the aptly named Event Horizon Telescope as the most sensitive participating site, increasing sensitivity by a factor of 10,” he said.

To add the signals from all the antennas, specialized electronics and computer equipment were built at the National Radio Astronomy Observatory’s Central Development Lab in Charlottesville, Virginia. These new circuit boards were installed into ALMA’s correlator, the supercomputer that combines the signals from the antennas.

During the upcoming observations, the signal from the phased array will be time-stamped and encoded by a dedicated atomic clock. This will allow the data to be shipped to a central processing center where it will be combined with identically timed signals from other telescopes.

The high-speed recorders that will capture the torrent of data flowing from the ALMA phased array were designed by the MIT Haystack Observatory. Software to run the new phasing system was developed by multiple institutions involved in the phasing project.

# # #

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of ESO, the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

Contact: Charles Blue
434-296-0314; cblue@nrao.edu

 

Credit: NRAO/AUI/NSF

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