Illuminating the Black Hole at the Center of the Galaxy
If you were to visit the center of our Milky Way galaxy, you would find a chaotic, high-energy, and extremely bright environment, with huge molecular clouds forming new stars at a prodigious rate. And at the very heart of it all lies a supermassive black hole known as Sagittarius A* (or Sgr A*)—four million times more massive than our sun.
Scientists have long theorized that such supermassive black holes exist at the center of virtually all large galaxies and have recently gathered evidence of their existence. But it was not until the last few years that they were able to see a supermassive black hole—more precisely, the area around the void of the black hole through which no light escapes. In 2019, an international astronomy consortium known as the Event Horizon Telescope (EHT) Collaboration published in The Astrophysical Journal the first-ever image captured of a black hole—one located in the distant M87 galaxy, 55 million light years away. A large group of astronomers, including a University of Massachusetts Amherst team led by Gopal Narayanan and F. Peter Schloerb, captured data for the image during an eight-day observing campaign in 2017 involving an array of eight radio telescopes positioned around the globe. In May 2022, the EHT consortium unveiled yet another remarkable image showing the Sgr A* black hole at the center of our own galaxy.
Though the two black holes are notably different in size—and, as a result, the orbital timescales at which surrounding gases swirl around them differs greatly—they appear strikingly similar in the images: brightly glowing donut-shaped rings around a dark center. Because it is impossible to see the black hole itself, which is completely dark, the images capture light emitting from just outside the innermost region, called the event horizon, surrounding the black hole, where even light travels in a trapped circle being bent by the black hole’s powerful gravity.
These amazing images, decades in the making, shed light on the basic physics of how galaxies form and evolve. They mark a major milestone in a long, ongoing effort at UMass Amherst and elsewhere to test the limits of current theories about the universe, and to expand understanding of what lies beyond our own planet, outside of what is visible to human eyes.
Seeing the Unseeable
Imaging the two black holes required special instruments to allow astronomers to “see the unseeable,” as National Science Foundation (NSF) Director France Córdova put it when the first black hole image was unveiled. Historically, astronomers have studied objects in the sky emitting light visible to the human eye, but after World War II, their field of vision was greatly expanded by the emergence of radio astronomy. An abundance of radio antennae left over from the war led to a boom in astronomers looking at the sky at radio wavelengths to see what they could see.
“Radio astronomy covers the range of the light spectrum from around 1 millimeter all the way out to meters in wavelength,” explained Schloerb. As technology was developed to make increasingly precise receivers, astronomers were able to use radio telescopes to observe great clouds of gas and dust in the galaxy where molecules were being formed.
“Millimeter wave astronomy became a way to study the formation of stars,” Schloerb said. “As millimeter-wave telescopes have grown in size, they have allowed us to see farther and farther away. We can now see star formation in distant galaxies, which helps us understand how galaxies form and evolve over time.”
A pioneer in millimeter wave astronomy, UMass Amherst provided groundbreaking thought leadership and trained a generation of up-and-coming millimeter wave astronomers. When Schloerb first arrived on campus as a post-doc in the late 1970s, UMass’s 14-meter diameter telescope was one of the largest in the world.
However, “By the 1980s, we were being surpassed by other facilities. We started thinking about what comes next, and decided to pursue an even bigger millimeter-wave telescope,” said Schloerb.
Initial funding for the project came in 1994 to begin designs, and UMass formed a partnership with the country of Mexico to build the telescope. By 1997, they settled on a site—the top of a 15,000-foot peak in the state of Puebla—and began construction. Ultimately, the Large Millimeter Telescope (LMT) seen below, 50 meters in diameter, became the biggest—and, therefore, the most sensitive—telescope of its kind in the world, with Schloerb serving as its UMass director. It is operated jointly by UMass Amherst and Mexico’s Instituto Nacional de Astrofísica, Óptica y Electrónica. Building it was the largest science project of any kind in Mexican history.
The U.S. astronomy community identified access to telescopes like the LMT as a priority in the 2010 Decadal Survey of Astronomers. In September 2020, UMass Amherst received a three-year, $5 million National Science Foundation (NSF) grant to provide support for the LMT and to offer access to it for astronomers from any U.S. institution.
In the early 2000s, as the LMT was under construction, radio astronomers began exploring the possibility of joining forces to try to capture a picture of the black hole at the center of the galaxy. While some were skeptical at first, said Schloerb, momentum began to build around 2006 or 2007.
“Once we figured out that a global array of millimeter-wave telescopes is needed to image the supermassive black hole, we tried to include all the available telescopes,” said Narayanan. “There was a lot of interest in the LMT because of its size and location, and it became the premier telescope to be added to this global array.”
And, thus, the EHT collaboration was formed.
The EHT’s mission officially began in 2013, with Narayanan serving as UMass’s principal investigator.
“I had very good confidence in the experimental procedures, but these had worked at much lower frequencies in the past. One of the biggest questions for me was whether we could actually make an array spanning the entire globe and work at such high frequencies,” he said. “It wasn’t clear to me that we would succeed.”
Success would require precise timing and frequency standards. Narayanan led a team in constructing two radio astronomy receivers used to collect data at the LMT. In the years that followed, the team at the LMT contended with numerous technical challenges, and the physical demands of working at 15,000 feet altitude.
“It took a long time, but working collaboratively to solve problems one by one, we managed to do it,” said Narayanan.
Science is all about testing the limits of existing theories to find the conditions that break them, helping us to understand the world better. Black holes are natural laboratories with the most extreme levels of gravity, offering a unique opportunity to probe aspects of gravity.
Following a successful “dress rehearsal” in January 2017 involving a subset of telescopes in the array, the entire EHT collaboration launched its campaign in March 2017. Eight radio telescopes, including the LMT in Mexico along with those stationed in Hawaii, Arizona, Chile, Europe, and the South Pole, linked together to form a single “earth-sized” virtual telescope, observing the black holes simultaneously.
“It was very intense, but it was like a dream campaign,” Narayanan recalled. “All the technical problems had been solved. Amazingly, the weather was perfect at eight different sites around the world, every day.” Though the group had allotted 10 days to capture the data, they were able to get what they needed during the first eight days.
In all, more than 300 astronomers worked on the campaign, including a team of about a dozen from UMass Amherst, comprised of many graduate students and early career astronomers.
Sandra Bustamante, an international graduate student from Mexico, first became involved in the project while pursuing her master's degree at INAOE, UMass's partner institution in Mexico in operating the LMT. Narayanan invited her to be part of the telescope support team for the EHT. Bustamante later went on to pursue a PhD at UMass Amherst, and to become an official collaborator in the EHT. She serves on the instrumentation work group and participated as online support in the 2021 campaign.
"I feel very lucky to have been at the right place at the right time and be part of this great team," Bustamante said. "At the time, I couldn’t imagine the huge impact that this work would have. Whenever I talk about this project, people seem to think that creating images of M87 and Sgr A* were the ultimate goals but, in reality, this is just the beginning. The EHT has opened a whole new way to study black holes, and we are working hard to continue to get better images and learn more."
The Images and What They Reveal
The 2017 observing campaign yielded data to image both the black hole in the distant M87 galaxy and Sgr A* in the Milky Way. The former was imaged first, in 2019, due to its much larger size and the relatively slow movement of the gas swirling around it.
In contrast, Sgr A* is around 2,000 times smaller, which causes the gas to swirl around it much faster, blurring the image.
“It’s like taking a photo of an ice dancer doing a pirouette. When she’s spinning very fast, it’s extremely difficult to capture a crisp still image of her,” Narayanan explained. The astronomers developed new techniques to produce a clear picture, and the result was published in a special issue of The Astrophysical Journal Letters in May 2022.
These two unprecedented images provide overwhelming evidence that the objects at the center of galaxies are, in fact, black holes, as astronomers have long believed, and may shed new light on the formation and evolution of galaxies. In addition, said Narayanan, “They both conform amazingly closely to the prediction of Einstein’s theory of general relativity.”
One of the fundamental theories of the universe, the theory of general relativity published by physicist Albert Einstein in 1915, holds that the observed gravitational effect between masses arises from their warping of spacetime by massive objects, and predicts the existence of black holes.
“The reason we study supermassive black holes is to see if we can prove Einstein wrong,” said Narayanan. “Science is all about testing the limits of existing theories to find the conditions that break them, helping us to understand the world better. Black holes are natural laboratories with the most extreme levels of gravity, offering a unique opportunity to probe aspects of gravity.”
Ultimately, he said, studying black holes could lead to new understanding of quantum effects as well as gravitational effects. And this could lead to practical applications, such as discovering new ways of harnessing energy sources.
“Every time there is a fundamental discovery in basic science, there are practical applications that follow,” Narayanan said. “That’s why scientists are always probing things that seem esoteric to the layperson.”
Looking to the Future
The UMass astronomers, together with their global collaborators, are forging ahead to expand understanding of black holes. Another major observation campaign was conducted in March 2022, this time linking 12 telescopes around the world.
A new initiative, the next generation EHT (ngEHT), is working to greatly expand the size of the EHT array in the future and produce even sharper images. Narayanan is building a multi-frequency receiver that can obtain data from multiple frequencies of light at the same time in order to increase the resolution and fidelity of the images, while Schloerb is working to improve performance of the LMT’s antenna during the daytime hours in order to collect data over longer periods on the source. Together, these two efforts would allow them to participate in the ngEHT array to produce the first movies of time-variable phenomena around supermassive black holes.
In the long run, Narayanan said, EHT plans to put telescopes in space. "That would increase the effective size of the array to be even larger than an earth-sized telescope, and allow us to greatly increase the sample of supermassive black holes to many tens of objects. In addition, we would be able to probe even closer to the event horizon, unveiling perhaps fundamental new physics.”
(Header image photo credit: INAOE)
This article was first published in June 2022.