Scientists may have found a ‘missing-link’ black hole ripping up and devouring a star

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Astronomers have discovered that an unusual optical flare is the result of a star being torn apart and devoured by a black hole – and what really sets this so-called tidal disruption event (TDE) apart is the fact that the black hole involved appears to be an example of an elusive “intermediate mass black hole”, a class of this object that has challenged astronomers for decades.

TDEs generally occur when stars venture too close to the supermassive black holes which are found at the heart of large galaxies, giving rise to the immense gravity of these cosmic titans simultaneously crushing the stellar body horizontally while stretching it vertically. This “spaghettification” creates a stellar noodle that wraps around the black hole. Some of the remnants are funneled toward the central black hole, while much of it is blasted out at near-light speeds as high-energy jets. These events can take hundreds of days or even years to resolve.

This optical flare, designated AT2022zod, was spotted in October 2022 and lasted a little over a month. It was assigned to the galaxy SDSS J105602.80+561214.7, located about 1.5 billion light years away. Earth. What was intriguing was the fact that the TDE occurred about 10,000 light years from the center of this galaxy, where its supermassive black hole resides. This was the first clue that this was the work not of a central supermassive black hole, but of a non-central intermediate-mass black hole.

“AT2022zod has the characteristics of a TDE, a flare we observe when a star is torn apart while interacting with a black hole. These events are, in general, not common, but since we expect a supermassive black hole at the center of almost all galaxies, TDEs should be observed at the center of their host galaxy,” team leader Kristen Dage from Curtin University, Australia, told Space.com. “However, AT2022zod is slightly non-nuclear and very short compared to previously observed TDEs, while remaining very energetic.”

When observed at such great distances, TDEs typically last hundreds of days, making AT2022zod’s month-long duration, from October 13 to November 18, highly unusual. “The combination of being hosted by an elliptical galaxy, harboring large populations of star clusters, while being non-nuclear and short-lived, intrigued us that this could be one of the elusive intermediate-mass black holes that could exist outside the center of the galaxy, and more importantly, open a new avenue for searching and studying them,” Dage continued.

Intermediate black holes as cosmic intermediaries

Supermassive black holes are thought to have masses millions or billions of times greater than those of supermassive black holes. sunwhile stellar-mass black holes, which form from dying massive stars, would have masses ranging from three to several hundred times the mass of the sun. This leaves a huge mass range between these two types of black holes in which the aptly named intermediate-mass black holes are thought to lie.

Since supermassive black holes are thought to grow via merger chains between increasingly massive black holes, it is reasonable to assume that intermediate-mass black holes play a key role in this growth process. This means that black holes in this mass range should be quite ubiquitous in the cosmos, but astronomers have had great difficulty discovering them.

“I think it’s really hard to overstate how bad we are at finding intermediate-mass black holes. We’re great at finding supermassive black holes, and thanks to the LIGO-Virgo-Kagra gravitational wave detectors, we’re getting better at finding stellar-mass black holes, but I could count on my hands the number of intermediate-mass black hole candidates that have reached some sort of consensus within the astronomy community,” said Dage. “So far, TDEs of intermediate black holes are known to exist, but they are very difficult to observe. They are mostly overshadowed by other activities in the central region of the galaxy.”

Astronomers can distinguish between TDEs caused by intermediate black holes and those generated when supermassive black holes tear apart stars because of where they occur and the duration of these events.

“With our current understanding of TDE behavior, we know that the duration of the event scales with the mass of the black hole, so all things being equal, a shorter time scale indicates lower black hole mass,” Dage said. “What convinced me that AT2022zod is special was when I compared it to other TDEs at similar distances or with similar host galaxies, and it didn’t match the same behavior.”

The discovery of this off-center TDE could also say more about the environment occupied by this intermediate-mass black hole. For example, it is quite obvious that TDEs are much more likely to occur in regions where stars are densely clustered. “If you’re not in some sort of star cluster, usually the central nuclear cluster of the host galaxy, then you simply won’t have a TDE, because the chances of any given star waltzing near the black hole are too low,” Dage said. This stellar density is found in the cores of galaxies, but there are also non-central regions of galaxies in which stars are also tightly packed together.

Failed supermassive black holes?

The team hypothesizes that this TDE occurred in a globular cluster or ultracompact dwarf (UCD) galaxy within SDSS J105602.80+561214.7 itself. Globular clusters and UCDs are densely populated conglomerates of old stars reaching the end of their lives.

“These systems are essentially black hole factories, and their crowded and dynamic systems provide opportunities for black holes to merge and grow in the intermediate mass range, especially during runaway stellar collisions,” Dage said. “When you combine this with the observational evidence for kinematic studies of black holes in UCDs, it makes them very compelling environments to host intermediate-mass black holes!”

The origins of UCDs are currently shrouded in mystery. These dense stellar regions could appear when two globular clusters approach, collide and merge, or the UCDs could be dwarfs. galaxies which have been stripped of their outer stars, leaving them as a compact, dense galactic core.

“These two different formation scenarios have very different implications for the evolution of black holes. If they are stripped cores, then they are ‘failed’ supermassive black holes, with a formation path similar to that of supermassive black holes and large galaxies,” Dage explained. “If it’s just large globular clusters, things could be completely different and dynamics play a critical 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 distant that the team can’t really tease out the nature of AT2022zod’s real environment. “We just know it’s in some sort of star cluster,” Dage said. “Personally, I would love it to be in a globular cluster, but based on what we know about closer systems, a UCD makes a lot of sense as a host in the nearby universe.”

She added that many studies of the physics of UCDs show that they host black holes in the mass range estimated for AT2022zod. This includes a system in the Milky Way called Omega Centauri, although Dage pointed out that there is still debate over whether this densely populated star cluster in our galaxy is a UCD or a globular cluster.

Although the environment of TDE AT2022zod may remain a mystery for the near future, the team’s research could provide a much-needed roadmap for the discovery of intermediate black holes, which will become particularly relevant when the Vera C. Rubin Observatory begins conducting its decade-long Legacy Survey of Space and Time (LSST).

“Rubin is poised to have a huge impact: it will provide incredibly sensitive 10-year optical coverage of millions of star clusters within 330 million light-years, and should be sensitive to TDE populations hosted in dense stellar environments,” Dage concluded. “We just need to make sure we’re looking in the right places, that we can follow up quickly to better understand the physics and the host system, and be able to interpret what we see.”

The team’s results are available on the paper repository website arXiv.