An ending with a special shine: The glow of a star under the deadly spell of "dark gravitational force" has given astronomers the clue to an intermediate black hole in the interior of a dwarf galaxy. The results could form the basis for further searches for possible specimens of this elusive category. Among other things, this could help to explore the suspected role of the intermediate versions in the formation of supermassive black holes, say the astronomers.
Their gravity warps space so much that not even light can escape. This spectacular property makes black holes some of the most fascinating objects in astronomy. The cosmic "darklings" are assigned to three main categories according to their mass: The topmost are the supermassive giants of hundreds of thousands to tens of billions of solar masses that sit in the centers of large galaxies. The bottom category is what are known as stellar black holes, formed by the supernova explosion of massive stars. In between are the previously enigmatic intermediate versions with thousands of solar masses, presumably caused by collisions. It is assumed that they could occur mainly in dwarf galaxies. There are candidates, but so far the intermediate black holes have been difficult to detect.
Mass in the mirror of deathlight
In the case of supermassive black holes, detection often relies on light emitted by gas disks around the gravitational giants. In addition, they make themselves felt when they devour a star that has gotten into their "whirlpool". Due to the enormous gravitational forces, a so-called "Tidal Disruption Event" (TDE) occurs in which the star is torn apart and lights up strongly. The course of this radiation burst can enable conclusions to be drawn about the mass, as was already evident from early investigations into supermassive black holes. An international team of scientists has now applied this TDE method to the case of a luminous star death that occurred in the center of a dwarf galaxy about 850 million light-years away from us.
The flare was recorded as part of the Young Supernova Experiment (YSE) - a survey of various astronomical facilities designed to detect cosmic explosions and transient astrophysical events. The scientists report that the data included the radiation released when the black hole began to engulf the star and what happened next. As they explain, the duration of the event provided a basis for calculations. "Mainly because the YSE data gave us so much early information about the event, we were finally able to draw conclusions about the mass of the black hole," says lead author Charlotte Angus from the Niels Bohr Institute in Copenhagen.
Reference to mean mass
The results show that the black hole that engulfed the star is less than a million to about 50,000 times the mass of the Sun. It could therefore be an intermediate black hole. "This result is significant because it shows that we can use tidal disruption events to find and study intermediate-mass black holes in dwarf galaxies," said co-author Ryan Foley of the University of California, Santa Cruz (UCSC). This could provide a basis for future studies of intermediate-sized black holes to better understand this elusive category.
As the researchers point out, it's not just about the middle class itself. Astronomers suspect that the supermassive black holes could have arisen from the middle-weight versions. "One of the biggest unanswered questions in astronomy right now is how supermassive black holes formed," says co-author Vivienne Baldassare of Washington State University in Pullman. It could be that the early Universe was full of small dwarf galaxies with intermediate-mass black holes. Over time, these dwarf galaxies merged or were swallowed up by more massive galaxies. Their nuclei could have united in the process. This merging process would then have produced the extreme giants we see today. "If we can understand the population of intermediate-mass black holes out there - how many there are and where they are - we can gain clues as to whether our theories about supermassive black hole formation are correct," says co-author Enrico Ramirez-Ruiz from UCSC.
Source: University of California – Santa Cruz, professional article: Nature Astronomy, doi: 10.1038/s41550-022-01811-y