Scientists hope to improve their understanding of the growth of supermassive black holes in massive galaxies by studying intermediate-mass black holes. After lurking undetected in a dwarf galaxy, an intermediate-mass black hole revealed itself to astronomers when it devoured an unlucky star that strayed too close. Known as a “tidal disruption event” or TDE, the violent shredding of the star produced a burst of radiation that briefly exceeded the combined starlight of the host dwarf galaxy. This observation could help scientists better understand the relationships between black holes and galaxies. The explosion was captured by astronomers with the Young Supernova Experiment (YSE), a survey designed to detect cosmic explosions and transient astrophysical events. An international team led by scientists at UC Santa Cruz, the Niels Bohr Institute at the University of Copenhagen and Washington State University reported the discovery in a paper published today (Nov. 10) in Nature Astronomy. “This discovery has generated widespread excitement because we can use tidal disruption events not only to find more intermediate-mass black holes in quiescent dwarf galaxies, but also to measure their mass,” said co-author Ryan Foley, assistant professor of astronomy and astrophysics at UC Santa Cruz who helped design the YSE survey. First author Charlotte Angus at the Niels Bohr Institute said the team’s findings provide a basis for future studies of medium-sized black holes. Astronomers have discovered a star being torn apart by a black hole in the galaxy SDSS J152120.07+140410.5, 850 million light-years away. The researchers pointed to NASA’s Hubble Space Telescope to examine the aftermath, called AT 2020neh, seen in the center of the image. Hubble’s ultraviolet camera saw a ring of stars forming around the core of the galaxy where AT 2020neh is located. Credits: NASA, ESA, Ryan Foley/UC Santa Cruz “The fact that we were able to capture this medium-sized black hole while it was devouring a star offered us a remarkable opportunity to detect what would otherwise have been hidden from us,” Angus said. “Furthermore, we can use the properties of the flare itself to better understand this elusive group of intermediate-mass black holes, which could account for the majority of black holes at the centers of galaxies.” Supermassive black holes are found at the centers of all large galaxies, including our Milky Way. Astronomers speculate that these massive beasts, millions or billions of times the mass of the sun, could have grown from smaller “intermediate mass” black holes with thousands to hundreds of thousands of solar masses. One theory of how such massive black holes assembled is that the early universe was rampant with small dwarf galaxies with intermediate-mass black holes. Over time, these dwarf galaxies would have merged or been swallowed up by larger galaxies, their nuclei each time combining to create the mass at the center of the growing galaxy. This merging process would eventually create the supermassive black holes seen today. “If we can understand the population of intermediate-mass black holes out there — how many there are and where they are — we can help determine whether our theories about the formation of supermassive black holes are correct,” said co-author Enrico Ramirez-Ruiz, professor. of Astronomy and Astrophysics at UCSC and Professor Niels Bohr at the University of Copenhagen. But do all dwarf galaxies have medium-sized black holes? “That’s hard to claim, because detecting intermediate-mass black holes is extremely difficult,” Ramirez-Ruiz said. Classical black hole hunting techniques, which search for active black hole power, are often not sensitive enough to reveal black holes at the centers of dwarf galaxies. As a result, only a small fraction of dwarf galaxies are known to host intermediate-mass black holes. Finding more medium-sized black holes with tidal disruption events could help settle the debate about how supermassive black holes form. “One of the biggest open questions in astronomy right now is how supermassive black holes form,” said co-author Vivienne Baldassare, a professor of physics and astronomy at the University of Washington. Data from the Young Supernova experiment allowed the team to detect the first signs of light as the black hole began eating away at the star. Capturing this initial moment was crucial to unlocking how big the black hole was, because the duration of these events can be used to measure the mass of the central black hole. This method, which until now has only been shown to work well for supermassive black holes, was first proposed by Ramirez-Ruiz and co-author Brenna Mockler at UC Santa Cruz. “This flare was incredibly fast, but because the YSE data gave us so much early information about the event, we were actually able to detect the mass of the black hole using it,” Angus said. Citation: “A rapidly accreting tidal disturbance event from a candidate intermediate-mass black hole” by CR Angus, VF Baldassare, B. Mockler, RJ Foley, E. Ramirez-Ruiz, SI Raimundo, KD French, K. Auchettl, H Pfister , C. Gall, J. Hjorth, MR Drout, KD Alexander, G. Dimitriadis, T. Hung, DO Jones, A. Rest, MR Siebert, K. Taggart, G. Terreran, S. Tinyanont, CM Carroll, L. DeMarchi, N. Earl, A. Gagliano, L. Izzo, VA Villar, Y. Zenati, N. Arendse, C. Cold, TJL de Boer, KC Chambers, DA Coulter, N. Khetan, CC Lin, EA Magnier , C Rojas-Bravo, RJ Wainscoat and R. Wojtak, November 10, Nature Astronomy.DOI: 10.1038/s41550-022-01811-y This study was based on data from observatories around the world, including the WM Keck Observatory in Hawaii, the Nordic Optical Telescope, UC’s Lick Observatory, NASA’s Hubble Space Telescope, the Gemini International Observatory, the Palomar Observatory, and the Pan- STARRS Survey at Haleakala Observatory.
