Rare, midsize black hole caught devouring a star – Science

When it comes to the size of black holes, there’s a conspicuous gap in the middle. Astronomers have discovered dozens of small ones and tens of gargantuan ones, but only a handful of midsize ones.
Now, researchers have added another potential midsize black hole to the pile, and it’s ravenous: In a distant dwarf galaxy, astronomers have caught this cosmic beast in the act of devouring a star and spewing out brightly glowing crumbs. If more such untidy eaters can be found this way, it could bolster a long-held theory: that midsize black holes are the seeds from which their supermassive cousins grow.
Knowing how many dwarf galaxies harbor midsize black holes “will become a breakthrough,” says Igor Chilingarian, an astrophysicist at Harvard University who was not involved in the study. “Not only will it answer the black hole seeding question,” he says, but could also help understand how galaxies form.
The midsize black hole was bagged by the Young Supernova Experiment (YSE), a collaboration of astronomers that is primarily looking for stars that explode at the end of their lives. The team uses Pan-STARRS, a pair of 1.8-meter telescopes in Hawaii, to look at the same patch of sky every few days; the hope is to catch a supernova explosion in the first hours or days after it starts.
But in June 2020 the astronomers caught something else in their net: a rapidly brightening object in a dwarf galaxy nearly 1 billion light-years away. “We were very, very lucky,” says lead author Charlotte Angus of the University of Copenhagen. “We just jumped on it.” They continued to observe the object, dubbed AT 2020neh, over the following days and weeks using several ground-based telescopes as well as the Hubble Space Telescope. Its light curve—how its brightness changes over time—peaked after just over 13 days and then began a long slow decline.
The shape of the light curve and features in the light’s spectrum didn’t match those from a supernova; it seemed more like a tidal disruption event (TDE), the light show put on when a giant black hole containing millions or even billions of solar masses rips apart a star, consuming some of it and spraying out the rest in a bright superheated arc.
But the object reached its peak brightness more than twice as fast as in a typical TDE. Theorists who model these events predict smaller black holes produce fast-peaking TDEs. Using such models, the team calculated that AT 2020neh’s light curve could have been produced by a black hole with a mass of between 100,000 and 1 million Suns, they report today in Nature Astronomy. “I would say this is the most likely scenario,” Chilingarian says. “We still know too little about these events to be 100% certain.”
Astronomers believe all normal size galaxies have a supermassive black hole at their hearts. But it’s an open question whether dwarf galaxies, such as the one in which AT 2020neh was found, all contain midsize black holes. Because dwarf galaxies are small and faint, “they’re very difficult to detect,” Angus says.
With their apparent midsize TDE, the YSE researchers have stumbled on a new way to detect midsize black holes in dwarf galaxies. If they can detect a large enough sample, they may find whether the size of central black holes grows in step with galaxy size, something already seen in larger galaxies. If that relationship extends from dwarf galaxies right up through large ones, it supports the idea that galaxies get big through the merger of smaller ones, as opposed to coalescing from one gigantic cloud of gas. How galaxies form and grow is one of the great unknowns in astrophysics, one that astronomers hope new sharp-eyed space telescopes such as the recently launched James Webb Space Telescope will shine light on.
The YSE project will likely make only a small contribution to that project; Angus estimates it may detect just a handful more dwarf galaxy TDEs. But when the Vera C. Rubin Observatory, a survey telescope with an 8.4-meter mirror, starts up next year, it will be able to see deeper into space over a wider area. That scope is expected to find as many as 80,000 TDEs in its 10-year survey, so prospects are good.
Daniel Clery is Science’s senior correspondent in the United Kingdom, covering astronomy, physics, and energy stories as well as European policy.
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