Beyond the Shadow of Doubt: UMass Amherst Astronomer Helps find Proof of a Persistent Black Hole Shadow

The Event Horizon Telescope (EHT) Collaboration, which includes Gopal Narayanan, research professor in astronomy at the University of Massachusetts Amherst, recently released new images of M87*, the supermassive black hole at the center of the galaxy Messier 87, using data from observations taken in April 2018. 

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With the participation of the newly commissioned Greenland Telescope and a dramatically improved recording rate across the array, the 2018 observations provide a view of the source independent from the first observations in 2017. A recent paper published in the journal Astronomy & Astrophysics presents new images from the 2018 data that reveal a familiar ring the same size as the one observed in 2017. This bright ring surrounds a deep central depression, “the shadow of the black hole,” as predicted by general relativity. However, the brightness peak of the ring has shifted by about 30º compared to the images from 2017, which is consistent with theoretical understanding of variability from turbulent material around black holes.

The EHT collaboration involves 11 telescopes located around the world and more than 300 researchers from Africa, Asia, Europe, and North and South America. The international collaboration is working to capture the most detailed black hole images ever obtained by creating a virtual Earth-sized telescope. 

In 2017, the EHT took the first image of a black hole. This object, M87*, is the beating heart of the giant elliptical galaxy Messier 87 and lives 55 million light years away from Earth. The image of the black hole revealed a bright circular ring, brighter in the southern part of the ring. Further analysis of the data also revealed the structure of M87* in polarized light, providing greater insight into the geometry of the magnetic field and the nature of the plasma around the black hole. The new era of black hole direct imaging has opened a new window into black hole astrophysics and allows astronomers to test the theory of general relativity at a fundamental level. 

The image of M87* taken in 2018 is remarkably similar to what the team saw in 2017. The bright ring is of the same size; there is a dark central region; and one side of the ring is brighter than the other. The mass and distance of M87* will not appreciably increase throughout a human lifetime, so general relativity predicts that the ring diameter should stay the same from year to year.

“A fundamental requirement of science is to be able to reproduce results,” says Keiichi Asada, an associate research fellow at Academia Sinica Institute for Astronomy and Astrophysics in Taiwan. “Confirmation of the ring in a completely new data set is a huge milestone for our collaboration and a strong indication that we are looking at a black hole shadow and the material orbiting around it.”

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This new observation was made possible because the EHT is under a cycle of continuous development. The Greenland Telescope joined the EHT for the first time in 2018, just five months after its construction was completed far above the Arctic Circle. This new telescope significantly improved the image fidelity of the EHT array, improving the coverage, particularly in the North-South direction. 

In addition, the Large Millimeter Telescope, co-run by UMass Amherst and Mexico’s Instituto Nacional de Astrofísica, Óptica y Electrónica, upgraded their instrument from a 32.5-meter diameter telescope to a 50-meter telescope. Since the LMT is centrally located in the array of EHT telescopes, the resulting increased sensitivity was an important contributor to the data collected in 2018. 

“Our astronomy lab at UMass Amherst also built a new dual-polarization sideband-separation receiver for carrying out EHT observations at the LMT,” says Narayanan. “This receiver was used for the first time in the 2018 EHT campaign at the LMT.” 

“Advancing scientific endeavors requires continuous enhancement in data quality and analysis techniques,” said Rohan Dahale, a Ph.D. candidate at the Instituto de Astrofísica de Andalucía (IAA-CSIC) in Spain. “The inclusion of the Greenland Telescope in our array filled critical gaps in our Earth-sized telescope. The 2021, 2022 and the forthcoming 2024 observations witness improvements to the array, fueling our enthusiasm to push the frontiers of black hole astrophysics.”

“One of the remarkable properties of a black hole is that its radius is strongly dependent on only one quantity: its mass,” said Dr. Nitika Yadlapalli Yurk, a former graduate student at the California Institute of Technology who is now a postdoctoral fellow at the Jet Propulsion Laboratory in California. “Since M87* is not accreting material (which would increase its mass) at a rapid rate, general relativity tells us that its radius will remain fairly unchanged over human history. It’s pretty exciting to see that our data confirm this prediction.”

While the size of the black hole shadow did not change between 2017 and 2018, the location of the brightest region around the ring did change significantly. The bright region rotated about 30º counterclockwise to settle in the bottom right part of the ring at about the 5 o’clock position. Historical observations of M87* with a less sensitive array and fewer telescopes also indicated that the shadow structure changes yearly but with less precision. While the 2018 EHT array still cannot observe the jet emerging from M87*, the black hole spin axis predicted from the location of the brightest region around the ring is more consistent with the jet axis seen at other wavelengths. 

“The biggest change, that the brightness peak shifted around the ring, is actually something we predicted when we published the first results in 2019,” said Britt Jeter, a postdoctoral fellow at Academia Sinica Institute for Astronomy and Astrophysics in Taiwan. “While general relativity says the ring size should stay pretty fixed, the emission from the turbulent, messy accretion disk around the black hole will cause the brightest part of the ring to wobble around a common center. The amount of wobble we see over time is something we can use to test our theories for the magnetic field and plasma environment around the black hole.”