Astronomers reveal Milky Way's giant black hole

Sagittarius A* (Sgr A*) at the center of our galaxy was observed by earth-sized Multiple telescopes, Multiple Radio astronomy telescopes combined EHT (Event Horizon telescope) and simultaneously with X-ray Observatories

First image of the black hole at the center of the Milky Way This is the first image of Sagittarius A* (or Sgr A* for short), the supermassive black hole at the center of our galaxy. It’s the first direct visual evidence of the presence of this black hole. It was captured by the Event Horizon Telescope (EHT), an array which linked together eight existing radio observatories across the planet to form a single “Earth-sized” virtual telescope. The telescope is named after the “event horizon”, the boundary of the black hole beyond which no light can escape. Although we cannot see the event horizon itself, because it cannot emit light, glowing gas orbiting around the black hole reveals a telltale signature: a dark central region (called a “shadow”) surrounded by a bright ring-like structure. The new view captures light bent by the powerful gravity of the black hole, which is four million times more massive than our Sun. The image of the Sgr A* black hole is an average of the different images the EHT Collaboration has extracted from its 2017 observations. Credit: EHT Collaboration

May 12, 2022 At simultaneous press conferences around the world, including at a National Science Foundation-sponsored press conference at the US National Press Club in Washington, D.C., astronomers have unveiled the first image of the supermassive black hole at the center of our own Milky Way galaxy.

"Until now, we didn't have the direct picture to prove that this gentle giant in the center of our galaxy is a black hole," Feryal Özel, an astrophysicist at the University of Arizona, said during a National Science Foundation news conference held Thursday (May 12). "It shows a bright ring surrounding the darkness, and the telltale sign of the shadow of the black hole."

The effort was made possible through the ingenuity of more than 300 researchers from 80 institutes around the world that together make up the EHT Collaboration. In addition to developing complex tools to overcome the challenges of imaging Sgr A*, the team worked rigorously for five years, using supercomputers to combine and analyze their data, all while compiling an unprecedented library of simulated black holes to compare with the observations.

This image of Sagittarius A*, and of the black hole in M87 before it, has been made possible through the magic of a technique known as Very Long Baseline Interferometry, which allows astronomers to combine data from radio telescopes all across the world as though they were one large telescope, effectively making the EHT the largest telescope on Earth.

At the time when the observations were made, the network consisted of eight telescopes (including one, the South Polar Telescope, that was too far south to study M87), although three more have since been added to the network. The eight-telescope configuration means that the EHT's maximum baseline — which is equivalent to a telescope's aperture — for observing Sagittarius A* was 10,700 kilometers (6,650 miles) across for an earth's hemisphere.

A comparison of Event Horizon Telescope views of the black holes at the center of the galaxy M87, on the left, and of the one in the Milky Way, at right. (Image credit: EHT Collaboration)

In 2019, the EHT made history in headlines when it succeeded in producing the first-ever image of the event horizon of a black hole, specifically the black hole at the center of the active elliptical galaxy Messier 87. At the same time as it gathered the data that became that image, the EHT also performed observations of Sagittarius A*, which is the name given to the Milky Way's supermassive black hole. However, producing an image of Sagittarius A* proved more difficult than for M87.

“We are very proud at CDL to have provided some critical technology to support this amazing discovery by the EHT collaboration,” said Bert Hawkins, Director of CDL, who explained the role of Band 6 and CDL in making the research and the results possible. “Our team contributed by installing a custom-built atomic clock on ALMA and reprogramming the ALMA correlator to make the telescope a phased array. This effectively turned the telescope into a single dish with an effective diameter of 85 meters– the largest component on the EHT. In addition, the mixers at the heart of the receivers on ALMA, the Submillimeter Telescope (SMT) in Arizona, the Large Millimeter Telescope (LMT) in Mexico, and the South Pole Telescope (SPT) in Antarctica were developed at CDL along with our partners at the University of Virginia.”

For one thing, Earth's moisture-laden atmosphere can absorb the submillimeter radio waves that the EHT observatories rely on. That sensitivity comes from the 1.3mm Band 6 receivers on the Atacama Large Millimeter/submillimeter Array (ALMA), designed by the Central Development Laboratory (CDL) at the US National Science Foundation’s National Radio Astronomy Observatory (NRAO), along with the X-ray data from Chandra, NuSTAR, and Swift, scientists in the EHT's 2017 campaign also obtained radio data from the East Asian very long-baseline Interferometer (VLBI) network and the Global 3 millimeter VLBI array; and infrared data from the European Southern Observatory's Very Large Telescope in Chile.

A representative shows the first image of the black hole at the center of the Milky Way at its location on the sky. (Image credit: ESO/José Francisco Salgado (josefrancisco.org), EHT Collaboration)

Moreover, gas and dust in the intervening 27,000 light-years between us and Sagittarius A* can scatter the submillimeter waves and blur the image. Lastly, whereas M87's black hole has a voracious appetite and appears bright because it is consuming a lot of gas, the flow of material onto Sagittarius A* is far more feeble, meaning it is much fainter.

Scientists are particularly excited to finally have images of two black holes of very different sizes, which offers the opportunity to understand how they compare and contrast. They have also begun to use the new data to test theories and models of how gas behaves around supermassive black holes. This process is not yet fully understood, but is thought to play a key role in shaping the formation and evolution of galaxies.

Black holes are the densest objects in the universe, and their gravity is irresistible, to the extent that within a certain distance of a black hole, not even light can escape. Scientists call this "point of no return" the event horizon.

The EHT is able to see light, in the form of radio waves, from hot gas swirling around the edge of the event horizon. The black hole feeds from the material within its immediate environment, whether gas clouds, asteroids or even stars that might wander too close and be ripped apart by gravitational tides.

“Now we can study the differences between these two supermassive black holes to gain valuable new clues about how this important process works,” said EHT scientist Keiichi Asada from the Institute of Astronomy and Astrophysics, Academia Sinica, Taipei. “We have images for two black holes — one at the large end and one at the small end of supermassive black holes in the Universe — so we can go a lot further in testing how gravity behaves in these extreme environments than ever before.”

Scientists in the large international collaboration compared the data from EHT, NASA's high-energy missions and the other telescopes to state-of-the-art computational models that consider factors such as Einstein's general theory of relativity, effects of magnetic fields, and predictions of how much radiation the material around the black hole should generate at different wavelengths. A visualization of one of these computer models is given in an accompanying video, which shows material rotating around, falling towards and being blasted away from the black hole

Visualization of Matter Near Sagittarius A* (Credit: Koushik Chatterjee (BHI, Harvard University) using code H-AMR (Liska et al. 2019).

The main panel of this graphic contains X-ray data from Chandra (blue) depicting hot gas that was blown away from massive stars near the black hole. Two images of infrared light at different wavelengths from NASA's Hubble Space Telescope show stars (orange) and cool gas (purple). These images are seven light years across at the distance of Sgr A*. A pull-out shows the new EHT image, which is only about 1.8 x 10-5 light years across (0.000018 light years, or about 10 light minutes).

Simulated images of Sgr A*. Left: a single snapshot image of a numerical simulation of Sgr A* that passes 10 out of the 11 observational criteria described in Paper V. Middle: the average of this simulation with time sampling that matches the EHT observational cadence on April 7. Right: representative image reconstruction using synthetic visibilities generated from the simulation in the adjacent panels (see Appendix H in Paper III). This image has been averaged across methodologies and reconstructed morphologies, as in Figure 3. Each panel is shown on a linear brightness scale that is normalized to its peak. (Image credit: EHT Collaboration)

SagrA* Blackhole Infographic Credit: NRAO/AUI/NSF, B. Saxton (NRAO) EHT Collaboration

Progress on the EHT continues: a major observation campaign in March 2022 included more telescopes than ever before. The ongoing expansion of the EHT network and significant technological upgrades will allow scientists to share even more impressive images as well as movies of black holes in the near future.

“These new results from EHT are exciting both because they show us how far astronomy has come already, and also because they confirm that there’s still so much out there we haven’t seen and haven’t yet been able to observe and study,” said Dr. Tony Beasley, Director of NRAO. “The antennas and instrumentation we design and develop at NRAO are making this progress possible, and we look forward to continuing to lead advances in radio astronomy that will uncover black holes and other phenomena lurking in the corners of the galaxy and the Universe.”

In 2021, NSF and the ALMA Board approved a multi-million dollar upgrade for the Observatory’s Band 6 receivers through the North American ALMA Development Program. The upgrade will increase the quantity and quality of science measured in wavelengths between 1.4mm and 1.1mm, which will provide research projects like those at EHT with better sensitivity than ever before, and ultimately more accurate and more efficient science results. In addition, NRAO’s Next Generation Very Large Array (ngVLA) received positive support from the Astro2020 decadal survey. Currently, in early-stage planning and development, the ngVLA will achieve high priority goals in astronomy and astrophysics and is slated to become the ultimate black hole hunting machine.

"The Event Horizon Telescope has captured yet another remarkable image, this time of the giant black hole at the center of our own home galaxy," said NASA Administrator Bill Nelson. "Looking more comprehensively at this black hole will help us learn more about its cosmic effects on its environment, and exemplifies the international collaboration that will carry us into the future and reveal discoveries we could never have imagined."

The EHT team's results are being published May 12 in a special issue of The Astrophysical Journal Letters. The multiwavelength results are mainly described in papers II and V.