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Midsize Black Hole Lurking In A Dwarf Galaxy Detected As It Began To Eat The Star

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When an unlucky star went too close to an intermediate-mass black hole lurking undiscovered in a dwarf galaxy, it revealed itself to astronomers. The shredding of the star, known as a “tidal disruption event” or TDE, caused a radiation flare that briefly outshone the combined stellar brightness of the dwarf galaxy’s host and could help scientists better comprehend the interactions between black holes and galaxies.

Astronomers caught the flare with the Young Supernova Experiment (YSE), a survey that looks for cosmic explosions and other short-lived astrophysical events. In an article published on November 10 in Nature Astronomy, an international team led by scientists from UC Santa Cruz, the Niels Bohr Institute at the University of Copenhagen, and Washington State University revealed the discovery.

According to coauthor Ryan Foley, “this discovery has created widespread excitement because we can use tidal disruption events not only to find more intermediate-mass black holes in quiet dwarf galaxies, but also to measure their masses.”

Charlotte Angus, the lead author from the Niels Bohr Institute, said that the team’s results can be used as a starting point for future research on midsize black holes.

“The fact that we were able to capture this midsize black hole whilst it devoured a star offered us a remarkable opportunity to detect what otherwise would have been hidden from us,” Angus adds. “What is more, we can use the properties of the flare itself to better understand this elusive group of middle-weight black holes, which could account for the majority of black holes in the center of galaxies.”

All big galaxies, including the Milky Way, have supermassive black holes at their centres. These enormous monsters, with masses millions or billions of times more than that of the sun, may have developed from smaller “intermediate-mass” black holes, with masses of tens of thousands to millions of solar masses.

According to one theory, the early cosmos was filled with several small dwarf galaxies that contained intermediate-mass black holes. These dwarf galaxies would have merged or been swallowed up by larger galaxies over time, with each merger increasing the mass of the center of the galaxy. This process of merging would lead to the supermassive black holes that we see today.

“If we can understand the population of intermediate-mass black holes out there—how many there are and where they are located—we can help determine if our theories of supermassive black hole formation are correct,” adds coauthor Enrico Ramirez-Ruiz.

But do all small galaxies have black holes about the same size?

“That’s difficult to assert, because detecting intermediate-mass black holes is extremely challenging,” Ramirez-Ruiz adds.

Traditional ways of looking for black holes, such as looking for ones that are getting food, are often not sensitive enough to find black holes in the hubs of dwarf galaxies. Because of this, we only know of a very small number of dwarf galaxies that have intermediate-mass black holes. Finding more medium-sized black holes with tidal disruption events could help end the debate about how supermassive black holes form.

“One of the biggest open questions in astronomy is currently how supermassive black holes form,” adds coauthor Vivienne Baldassare.

Using data from the Young Supernova Experiment, the team was able to see the first signs of light as the black hole began to eat the star. Getting a picture of this first moment was key to figuring out how big the black hole was. The length of these events can be used to measure the mass of the central black hole. Before now, this method had only been shown to work well for supermassive black holes. Ramirez-Ruiz and coauthor Brenna Mockler at UC Santa Cruz were the first to suggest it.

“This flare was incredibly fast, but because our YSE data gave us so much early information about the event, we were really able to pin down the mass of the black hole using it,” Angus adds.

Source: 10.1038/s41550-022-01811-y

Image Credit: NASA, ESA, Ryan Foley/UC Santa Cruz

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