The world's most powerful space telescope reveals 'overmassive' black holes and a smoking gun for a black hole merger, providing new insights into galaxy evolution
Studies:
A JWST View of the Overmassive Black Hole in NGC 4486B (DOI: 10.3847/2041-8213/adf728)
A Supermassive Black Hole in a Diminutive Ultracompact Dwarf Galaxy Discovered with JWST/NIRSpec+IFU (DOI: 10.3847/2041-8213/ae028e)
JWST Observations of the Double Nucleus in NGC 4486B: Possible Evidence for a Recent Binary SMBH Merger and Recoil (DOI:10.3847/2041-8213/ae52ef)
An international research team, including contributors from the University of Michigan, have published three new studies that reveal new clues about how galaxies are built and torn apart.
Published in The Astrophysical Journal Letters, the studies focus on two dwarf galaxies in the Virgo Cluster, a cluster with thousands of member galaxies located about 55 million light-years from Earth. The team found that the two galaxies have "overmassive" black holes-that is, black holes that make up a surprisingly large fraction of the galaxy's total mass.
In both cases, the team believes that this is because the galaxies have had a large fraction of their stars ripped away by interactions with the neighboring galaxies and dark matter in the Virgo Cluster. The team also found strong evidence that one of the black holes actually began as two separate black holes that collided and merged.
"The results tell us that there is a fairly broad diversity of black holes in dwarf galaxies," said Monica Valluri, a senior author of the studies and U-M research professor of astronomy. "They're telling us that black holes in dwarf galaxies, especially those in the dense environment of the Virgo Cluster, are very interesting."
All of the studies make use of NASA's James Webb Space Telescope, the world's most powerful infrared space telescope, which was developed in partnership with the Canadian Space Agency and the European Space Agency. The studies are based on observations from a Canadian-led observing program using an instrument on JWST known as the Near Infrared Spectrograph, or NIRSpec.
That's not where black holes belong
One of the tricky things about studying black holes is that they don't emit or reflect light, which is what telescopes like JWST collect to make their observations. But scientists can see how visible objects, notably stars, near black holes behave. Researchers have learned how to use those observations to extract a black hole's properties, such as its mass and how it's moving.
For decades, Valluri has been writing computer code that uses the motion of stars at the centers of galaxies to determine the masses of their supermassive black holes, or SMBHs. It was code developed with Eugene Vasiliev at the University of Surrey in the U.K. that enabled these new studies.
Two of the studies focused on a black hole in a compact galaxy known as NGC 4486B, which has an SMBH that is about 360 million times the mass of the sun. NGC 4486B has presented a particularly interesting puzzle for astronomers since the 1990s.
Back then, observations from the Hubble Space Telescope revealed that NGC 4486B had not one central bright spot, or nucleus, as in most galaxies, but a double nucleus. Scientists postulated that this was because of a disk of material surrounding NGC 4486B's SMBH that was elliptical, or eccentric, rather than circular. Another observation-a one-dimensional spectrum taken by a telescope in Hawaii-also showed that the galaxy's black hole was off-center.
"In most galaxies where we see a black hole, it's bang-on in the center of the galaxy.
Valluri said. "You can see clearly that it's off-center in NGC 4486B."
Now, the much more detailed view that JWST has has allowed the team to explain the off-center black hole and the eccentric disk. That explanation involves a black hole merger in the recent past: Two black holes were engaged in a cosmic dance before eventually colliding and merging into one.
"There have been a number of predictions about what galaxies that have experienced black hole mergers should look like in the aftermath. We believe this discovery is a smoking gun for that," Valluri said.
Before the collision, the two black holes would have been spinning around each other. That does two things that give the researchers confidence they're now seeing the aftermath of a merger. First, the black holes' spinning around each other would launch nearby stars out of the center of the galaxy as they get closer. This produces a stellar deficit, which is seen in NGC 4486B as a core. Secondly, when black holes merge, they emit gravitational waves.
Theory predicts that the black holes' pre-collision configuration could introduce asymmetry to the process such that, when they crash into each other, the waves would shoot out preferentially in a certain direction. Theory predicts that that would create a recoil that kicks the resulting single black hole out of the galaxy's center in the opposite direction.
The new JWST-NIRSpec observations provide much more precise 2-dimensional data on the velocities of stars around the black hole, allowing the team to better model the distribution of star trajectories around the black hole. This analysis supports the hypothesis that the current black hole resulted from a merger and was kicked out of the center and is still in the process of settling back in. There are very few ways other than a gravitational wave recoil kick that could move such a massive black hole out from the center of its galaxy.
"What makes this system particularly interesting is that the double nucleus appears to preserve the dynamical signature of a recoil kick following a black hole merger," said Behzad Tahmasebzadeh, who was a postdoctoral fellow at the University of Michigan when he led two of the three studies. He is now a postdoctoral fellow at Villanova University.
"By analyzing the motions of stars in the galaxy's center, we can trace how the central structure was disturbed after this event," Tahmasebzadeh said. "Observations like this provide rare insight into how black hole mergers can reshape the inner regions of galaxies."
Given enough time, this black hole will sink back to the center of the galaxy, Valluri said, which means the collision was fairly recent in astronomical time-perhaps only about 30 million to 80 million years ago (for comparison, galaxies evolve on timescales of billions of years).
"The discovery of this off-center, post-merger SMBH, which is only possible due to the capabilities of JWST, offers a rare opportunity to study how black hole mergers play into their growth over cosmological timescales," said Matt Taylor, professor at the University of Calgary and lead author of the third study. "Catching this one so soon after the act is also a unique opportunity to see how the mergers sculpt the innermost regions of their host galaxies."
The hole's a big part of the whole
In a separate publication about NGC 4486B, the team reported the galaxy had another cosmic idiosyncrasy: The mass of its black hole accounts for somewhere between 4% to 13% of the galaxy's total mass. For comparison, that percentage in a typical galaxy is closer to 0.1%, Valluri said.
In the third study, the team observed a different galaxy in the Virgo Cluster-the ultracompact dwarf galaxy called UCD736-which also had an "overmassive" black hole at its core. Although the mass of UCD736's black hole was about 180 times smaller than that of NGC 4486B's, it still accounted for 8% of the mass of the entire system.
This discovery strongly suggests that UCD736 was once a much larger galaxy whose outer layers were stripped away by the harsh dynamical environment of the Virgo Cluster. What remains today is likely just the nucleus-and its central black hole. This finding also represents the detection of a supermassive black hole in the most compact stellar system identified to date.
"The NIRSpec instrument on JWST has given us the unique opportunity to search for SMBHs within smaller and fainter galaxies than was possible with ground-based telescopes," said Solveig Thompson, doctoral student at the University of Calgary and member of the discovery team. "We also find parallels between UCD736 and NGC 4486B where they both host unusually large SMBHs for their size, suggesting they both started much larger than they are today and evolved along the same stripping pathway in the Virgo Cluster."
Together, the two galaxies demonstrate how studying the black holes at a galaxy's heart can act as a kind of cosmic time capsule, preserving clues about past mergers, environmental effects and the violent processes that shape galaxies. The team plans to continue investigating similar systems in the Virgo Cluster and beyond to better test models of galaxy evolution and to help solve more mysteries, while uncovering new ones.
