Astronomers have discovered that the unusual optical flare is the result of a star being torn apart and swallowed by a black hole. And what makes this so-called tidal disruption event (TDE) stand out is the fact that the black hole involved appears to be an example of the elusive “intermediate-mass black hole.” This object has posed a challenge to astronomers for decades.
TDE usually occurs when a star gets too close to Earth. supermassive black hole Located at the center of a large galaxy, resulting in a huge gravity These cosmic giants simultaneously stretch star bodies vertically and crush them horizontally. This “spaghettification” creates a star noodle that wraps around the black hole. Some of the debris is fed into the central black hole, but much of it is blown away as high-energy jets at near-light speeds. These events can take hundreds of days or even years to disappear.
“AT2022zod has the signature of a TDE, a flare observed when a star is torn apart by an interaction with a black hole. Although these phenomena are generally uncommon, we expect a supermassive black hole to exist at the center of almost every galaxy, so we expect TDEs to be observed at the center of its host galaxy,” team leader Kristen Dage from Australia’s Curtin University told Space.com. “However, AT2022zod is slightly out of the nucleus, and although it is very short compared to previously observed TDEs, it still has high energy.”
TDEs typically last hundreds of days when observed at such great distances, so it is highly unusual for AT2022zod to last for a month, from October 13th to November 18th. “The combination of being non-nuclear and short-lived, while being hosted in an elliptical galaxy famous for being home to massive star clusters, intrigues us that this could be one of the elusive intermediate-mass black holes that may exist outside the galactic center, and more importantly opens new avenues to explore and study them,” Dage continued.
A supermassive black hole is thought to have a mass millions or billions of times that of a black hole. solarOn the other hand, stellar-mass black holes formed from dying massive stars are thought to have masses between three and several hundred times the mass of the Sun. This leaves a huge mass range between these two types of black holes, within which the aptly named intermediate-mass black holes are thought to exist.
Since supermassive black holes are thought to grow through merging chains between increasingly massive black holes, it is reasonable to speculate that intermediate-mass black holes play an important role in this growth process. This means that black holes in this mass range should be fairly ubiquitous in the universe, but astronomers have had a very difficult time finding them.
“I think it’s really hard to overstate how bad we are at finding intermediate-mass black holes. We’re good at finding supermassive black holes, and thanks to the LIGO-Virgo-Kagra gravitational-wave detector, we’re getting better at finding stellar-mass black holes, but you can count on two hands the number of intermediate-mass black hole candidates that have reached some kind of consensus within the astronomical community,” Dage said. “Until now, TDEs from intermediate black holes have been known to exist, but they have been very difficult to observe. They are most often overshadowed by other activity within the central regions of galaxies.”
Astronomers can distinguish between TDEs caused by intermediate black holes and those that occur when a supermassive black hole tears apart a star, depending on where they occur and how long these events last.
“Our current understanding of TDE behavior is that the event duration scales with the mass of the black hole, so all other things being equal, shorter time scales indicate lower mass black holes,” Dage said. “What made me think AT2022zod was special was when I compared it to other TDEs at similar distances and similar host galaxies, which did not exhibit the same behavior.”
The discovery of this off-center TDE could reveal more about the environment occupied by this intermediate-mass black hole. For example, it is clear that TDEs are much more likely to occur in regions where stars are densely populated. “TDE will not occur unless it is in some type of star cluster, typically the central core cluster of the host galaxy, because the probability of a particular star dancing around near the black hole is too low,” Dage said. This density of stars is seen at the center of the galaxy, but there are also regions outside the center of the galaxy where stars are densely packed.
Did the supermassive black hole fail?
The researchers theorize that this TDE originated in a globular cluster or ultracompact dwarf galaxy (UCD) within SDSS J105602.80+561214.7 itself. Both globular clusters and UCDs are dense collections of ancient stars that have reached the end of their lives.
“These systems are essentially black hole factories, and their crowded dynamical systems provide opportunities for black holes to coalesce and grow into intermediate mass ranges, especially through collisions of runaway stars,” Dage said. “Combining this with observational evidence from kinematic studies of black holes in the UCD makes the UCD a very convincing environment to host intermediate-mass black holes.”
The origin of UCD is currently shrouded in mystery. These dense stellar regions can occur when two globular clusters are attracted or collide and merge, or when the UCD is a dwarf. galaxy The outer stars have been removed, leaving behind a compact, dense, stripped galactic nucleus.
“These two different formation scenarios have very different implications for black hole evolution. If the core is stripped, the supermassive black hole is a ‘failed’ supermassive black hole, and the formation paths to supermassive black holes and large galaxies are similar,” Dage explained. “If they were just large globular clusters, the situation could be quite different, and dynamics play an important role in the formation and evolution of black holes.”
Dage said scientists know that elliptical galaxies host both globular cluster star systems and UCDs, but in this case the host galaxy is so far away that the research team cannot fully tease out the nature of AT2022zod’s actual environment. “All we know is that it’s in some kind of star cluster,” Dage said. “Personally, I’d be very happy if it were in a globular cluster, but from what we know about closer star systems, UCD makes a lot of sense as a nearby cosmic host.”
She added that many studies on the physics of UCD have shown that it hosts a black hole in the mass range estimated to be AT2022zod. This includes the Milky Way called Omega Centauri, but Dage pointed out that there is still debate as to whether this dense cluster in our galaxy is a UCD or a globular cluster.
Although TDE AT2022zod’s environment may remain a mystery for some time to come, the team’s research could provide a long-awaited roadmap for discovering intermediate black holes. Vera C. Rubin Observatory begins conducting the 10-year Legacy Study of Space and Time (LSST).
“Rubin is poised to make such a major impact. Rubin can provide incredibly sensitive optical coverage of millions of star clusters within 330 million light-years for 10 years, and should also be sensitive to TDE populations hosted in dense stellar environments,” Dage concluded. “All we need is to make sure we’re looking in the right places, have a deeper understanding of the physics and the host system, and be able to follow up quickly so we can interpret what we see.”
The team’s results are available on the paper repository site arXiv.