Mystery of the Missing Supermassive Black Hole –“The Galaxy is Ten Times the Diameter of the Milky Way” (Weekend Feature)

black hole

 

The story: In April of 2019, Space Telescope Science Institute astronomer Marc Postman said that when he first first saw the Hubble image of the monster elliptical galaxy galaxy in the Abell 2261 galaxy cluster with a core bigger and brighter than any seen before, he knew immediately that something was odd, that its central core embraced a mystery never before seen. Spanning a little over one million light-years, the galaxy is about ten times the diameter of our own, and itself massive, Milky Way galaxy.

Enormous, Diffuse Central Region

The bloated galaxy is a member of an unusual class of galaxies filled with a fog of starlight where there would normally be a concentrated peak of light around a central supermassive black hole. The central region is much larger than astronomers would expect for the galaxy’s size, with a bloated core is more than three times larger than the center of other very luminous galaxies.

Since the mass of a central black hole usually tracks with the mass of the galaxy itself, reports the Chandra Space Observatory, astronomers expect the galaxy (image below) in the center of Abell 2261 to contain a supermassive black hole that rivals the heft of some of the largest known black holes in the Universe.

In short, the supermassive object that governs galaxy evolution was missing.

“The core was very diffuse. The challenge was then to make sense of all the data, given what we knew from previous Hubble observations, and come up with a plausible explanation for the intriguing nature of this particular galaxy,” said Postman. The Abell 2261 cluster is part of a multi-wavelength survey, led by Postman, called the Cluster Lensing And Supernova survey with Hubble (CLASH). The survey probes the distribution of dark matter in 25 massive galaxy clusters.

 

Abell 2261

The authors concluded that either there is no black hole at any of these locations, or that it is pulling material in too slowly to produce a detectable X-ray signal.

“The Unexpected”–At the Heart of a Monster Elliptical Galaxy (Weekend Feature)

“Expecting to find a black hole in every galaxy is sort of like expecting to find a pit inside a peach,” added astronomer Tod Lauer of the National Optical Astronomy Observatory. “With this Hubble observation, we cut into the biggest peach and we can’t find the pit. We don’t know for sure that the black hole is not there, but Hubble shows that there’s no concentration of stars in the core.”

Still No Evidence…

Despite continued searches with the Chandra X-ray Observatory and Hubble Space Telescope, astronomers have as yet found no evidence that a distant black hole estimated to weigh between 3 billion and 100 billion times the mass of the Sun is anywhere to be found. Using Chandra data obtained in 1999 and 2004 astronomers had already searched the center of Abell 2261’s large central galaxy for signs of a supermassive black hole. They looked for material that has been superheated as it fell towards the black hole and produced X-rays, but did not detect such a source.

Ejected by a Collision?

With new, longer Chandra observations obtained in 2018, a team led by Kayhan Gultekin from the University of Michigan conducted a deeper search for the black hole. They also considered an alternative explanation, in which the black hole was ejected from the host galaxy’s center. This violent event may have resulted from two galaxies merging to form the observed galaxy, accompanied by the central black hole in each galaxy merging to form one enormous black hole.

When black holes merge, they produce ripples in spacetime called gravitational waves. If the huge amount of gravitational waves generated by such an event were stronger in one direction than another, the theory predicts that the new, even more massive black hole would have been sent careening away from the center of the galaxy in the opposite direction. This is called a recoiling black hole.

Is the Story Embedded in Gravitational Waves?

Astronomers have not found definitive evidence for recoiling black holes and it is not known whether supermassive black holes even get close enough to each other to produce gravitational waves and merge; so far, astronomers have only verified the mergers of much smaller black holes. The detection of recoiling supermassive black holes would embolden scientists using and developing observatories to look for gravitational waves from merging supermassive black holes.

The galaxy at the center of Abell 2261 is an excellent cluster to search for a recoiling black hole because there are two indirect signs that a merger between two massive black holes might have taken place. First, data from the Hubble and Subaru optical observations reveal a galactic core — the central region where the number of stars in the galaxy in a given patch of the galaxy is at or close to the maximum value — that is much larger than expected for a galaxy of its size. The second sign is that the densest concentration of stars in the galaxy is over 2,000 light years away from the center of the galaxy, which is strikingly distant.

These features were first identified by Postman from Space Telescope Science Institute (STScI) and collaborators in their earlier Hubble and Subaru images, and led them to suggest the idea of a merged black hole in Abell 2261.

Recoiling Gargantuan Black Hole?

During a merger, the supermassive black hole in each galaxy sinks toward the center of the newly coalesced galaxy. If they become bound to each other by gravity and their orbit begins to shrink, the black holes are expected to interact with surrounding stars and eject them from the center of the galaxy. This would explain Abell 2261’s large core. The off-center concentration of stars may also have been caused by a violent event such as the merger of two supermassive black holes and subsequent recoil of single, larger black hole that results.

Even though there are clues that a black hole merger took place, neither Chandra nor Hubble data showed evidence for the black hole itself. Gultekin and most of his co-authors, led by Sarah Burke-Spolaor from West Virginia University, had previously used Hubble to look for a clump of stars that might have been carried off by a recoiling black hole. They studied three clumps near the center of the galaxy, and examined whether the motions of stars in these clumps are high enough to suggest they contain a ten billion solar mass black hole. No clear evidence for a black hole was found in two of the clumps and the stars in the other one were too faint to produce useful conclusions.

They also previously studied observations of Abell 2261 with the NSF’s Karl G. Jansky Very Large Array. Radio emission detected near the center of the galaxy showed evidence that supermassive black hole activity had occurred there 50 million years ago, but does not indicate that the center of the galaxy currently contains such a black hole.

 

Abell 2261

They then turned to Chandra to look for material that had been superheated and produced X-rays as it fell towards the black hole. While the Chandra data did reveal that the densest hot gas was not in the center of the galaxy, they did not reveal any possible X-ray signatures of a growing supermassive black hole — no X-ray source was found in the center of the cluster, or in any of the clumps of stars, or at the site of the radio emission.

The Conclusion

The authors concluded that either there is no black hole at any of these locations, or that it is pulling material in too slowly to produce a detectable X-ray signal.

Postman’s team admits that the ejected black-hole scenario may sound far-fetched, “but that’s what makes observing the Universe so intriguing—sometimes you find the unexpected,” he says.

“This is a system that’s interesting enough that it pushes against a lot of questions,” Lauer added. “We have thought an awful lot about what black holes do. But we haven’t been able to test our theories. This is an interesting place where a lot of the ideas we’ve had can come together and can be tested, fairly exotic ideas about how black holes may interact with each other dynamically and how they would affect the surrounding stellar population.”

So the mystery of this gigantic black hole’s location continues. Although the search was unsuccessful, concludes the Chandra Observatory, hope remains for astronomers looking for this supermassive black hole in the future. Once launched on October 31, 2021, the James Webb Space Telescope may be able to reveal the presence of a supermassive black hole in the center of the galaxy or one of the clumps of stars. If Webb is unable to find the black hole, then the best explanation is that the black hole has recoiled well out of the center of the galaxy. So far, a unique phenomena in the universe.

The Daily Galaxy, Jake Burba, via Arxiv.orgChandra X-Ray Observatory and Hubble Site