Webb reveals metal-rich gas around Milky Way’s central black hole

The gas orbiting Sagittarius A (Sgr A*), the supermassive black hole at the center of the Milky Way galaxy, is rich in metals, researchers at McGill University’s Trottier Space Institute have found. The metal enrichment probably resulted from stars that exploded and released material into the black hole’s surroundings over time, they said.

The team also learned this gas is similar in composition to many of the stars in the black hole’s orbit, suggesting the gas and stars may have formed together.

Sgr A* is closer to Earth, and therefore easier to see, than any other supermassive black hole. These findings could advance understanding of how metals are distributed around supermassive black holes in more distant galaxies, as well as the processes that supplied Earth with many of these elements, the researchers suggested.

In astronomy, metals include any element (e.g., carbon, oxygen, iron or gold) heavier than helium and hydrogen. Their presence influences how quickly gases cool, which in turn influences how readily the gas can be drawn into a black hole, the researchers said.

“We know there’s a black hole at the center of our galaxy pulling material into itself, but we see that it’s eating a lot less material than other supermassive black holes, and more slowly,” said Nicole Ford, a Ph.D. student in professor Daryl Haggard’s research group in the Department of Physics and lead author of the paper published in The Astronomical Journal.

“Part of understanding why this is happening means looking at the properties of the materials available for it to consume, including gas, dust and the stars orbiting around it.”

Data from the James Webb telescope

The study was conducted with data gathered using the James Webb Space Telescope (JWST), launched five years ago. The telescope measures infrared light over a wide range of wavelengths with unprecedented spectral and spatial resolution; this allowed the team to distinguish the heat signatures of multiple gas clouds orbiting near the black hole. It also enabled them to precisely measure the metal composition of each gas cloud.

The findings complement another recent study led by Banting postdoctoral fellow Mayura Balakrishnan published in The Astrophysical Journal that tested whether the gas preserved evidence of previous bursts of energy from the black hole. The results found that the gas emission is shaped by local conditions, making it difficult to isolate the black hole’s influence. Rather than ruling out past outbursts, this shows that the surrounding gas cannot provide a reliable history of the black hole’s activity.

Ford said that understanding how the black hole attracts material will require comparing these observations with simulations of how gas evolves over longer timescales.

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