An international research team, including contributors from the University of Michigan, published three studies on Apr. 6 revealing new insights into the formation and evolution of galaxies in the Virgo Cluster. The studies, appearing in The Astrophysical Journal Letters, focused on two dwarf galaxies with unusually large black holes at their centers.
The findings are significant because they provide evidence that some dwarf galaxies have “overmassive” black holes—black holes that account for a much larger portion of a galaxy’s mass than is typical. Researchers believe these oversized black holes are due to the galaxies losing much of their stellar material through interactions with neighboring galaxies and dark matter within the cluster.
Monica Valluri, a senior author and University of Michigan research professor of astronomy, said, “The results tell us that there is a fairly broad diversity of black holes in dwarf galaxies. They’re telling us that black holes in dwarf galaxies, especially those in the dense environment of the Virgo Cluster, are very interesting.”
The studies used data from NASA’s James Webb Space Telescope (JWST), specifically its Near Infrared Spectrograph instrument as part of a Canadian-led observing program. Two papers focused on NGC 4486B—a compact galaxy with a supermassive black hole about 360 million times more massive than our sun. Previous observations had shown an unusual double nucleus and an off-center position for this black hole.
“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.” New JWST data suggest this may be due to two former central black holes merging and creating gravitational waves powerful enough to shift their position away from the center—a rare phenomenon predicted by theory but seldom observed.
Behzad Tahmasebzadeh, formerly at University of Michigan and now at Villanova University, said: “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.” He added: “By analyzing the motions of stars in the galaxy’s center, we can trace how the central structure was disturbed after this event. Observations like this provide rare insight into how black hole mergers can reshape the inner regions of galaxies.”
Matt Taylor from University of Calgary noted: “The discovery … offers a rare opportunity to study how black hole mergers play into their growth over cosmological timescales.”
A separate publication found NGC 4486B’s central supermassive black hole accounts for between four percent and thirteen percent of its total mass—far higher than average for most galaxies according to Valluri. Another studied ultracompact dwarf galaxy (UCD736) showed similar characteristics; its smaller but still significant central supermassive black hole made up eight percent its total mass.
Solveig Thompson from University of Calgary said: “We also find parallels between UCD736 and NGC 4486B where they both host unusually large SMBHs for their size…” Both cases suggest these systems were once larger before being stripped down by interactions within Virgo Cluster.
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