The Monster in the Middle of the Milky Way Is…Spinning Slowly?

Cambridge, MA - The monstrous black hole at the center of the Milky Way galaxy—now of Nobel Prize fame—is proving yet again to be stranger than fiction. New research from scientists at the Center for Astrophysics | Harvard & Smithsonian (CfA), and the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) at Northwestern University has revealed that the supermassive black hole at the center of the Milky Way galaxy is not spinning much, providing more evidence that it is unlikely to have a jet. The paper is published in The Astrophysical Journal Letters.

Supermassive black holes like SgrA*—the monstrous black hole at the center of the Milky Way galaxy—are characterized by just two numbers: mass and spin, but have a critical influence on the formation and evolution of galaxies. According to Dr. Avi Loeb, Frank B. Baird Jr. Professor of Science at Harvard and CfA astronomer, and co-author on the research, "black holes release a huge amount of energy that removes gas from galaxies and therefore shapes their star formation history."

While scientists know that the mass of central black holes has a critical influence on their host galaxy, measuring the impact of their spin isn't easy. As Loeb puts it, "the effect of black hole spin on the orbits of nearby stars is subtle and difficult to measure directly."

To get a better understanding of how SgrA* has impacted formation and evolution of the Milky Way, Loeb and Dr. Giacomo Fragione, of CIERA, studied instead the stellar orbits and spatial distribution of S-stars—the closest stars orbiting SgrA* and traveling at a speed of up to a few percent of the speed of light—to constrain, or place limits on the spin of the black hole. "We concluded that the supermassive black hole in the center of our galaxy is spinning slowly," said Fragione. "This can have major implications for the detectability of activity in the center of our galaxy and the future observations of the Event Horizon Telescope."

The S-stars appear to be organized into two preferred planes. Loeb and Fragione showed that if SgrA* had a significant spin, the preferred orbital planes of the stars at birth would become misaligned by the present time. "For our study we used the recently discovered S-stars to show that the spin of the black hole SgrA* must be smaller than than 10-percent of its maximal value, corresponding to a black hole spinning at the speed of light," said Loeb. "Otherwise, the common orbital planes of these stars would not stay aligned during their lifetime, as seen today."

The results of the research also point to another important detail about SgrA*: it is unlikely to have a jet. "Jets are thought to be powered by spinning black holes, which act as giant flywheels," said Loeb, with Fragione adding that, "Indeed there is no evidence of jet activity in SgrA*. Upcoming analysis of data from the Event Horizon Telescope will shed more light on this issue."

The find was published just days before the announcement of the 2020 Nobel Prize in Physics, which was awarded in part to scientists Reinhard Genzel and Andrea Ghez for their ground-breaking research which demonstrated that SgrA* is a black hole. "Genzel and Ghez monitored the motion of stars around it,” said Loeb. "They measured its mass but not its spin. We have derived the first tight limit on SgrA*'s spin," adding that the find wouldn’t be possible without Genzel and Ghez's original Nobel Prize-winning work.

This work was supported in part by a CIERA Fellowship at Northwestern University, and Harvard's Black Hole Initiative, which is funded by grants from the John Templeton Foundation and the Gordon and Betty Moore Foundation.

Reference: "An upper limit on the spin of SgrA based on stellar orbits in its vicinity," G. Fragione and A. Loeb, The Astrophysical Journal Letters.

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Headquartered in Cambridge, Mass., the Center for Astrophysics | Harvard & Smithsonian (CfA) is a collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

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