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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