Harvard Smithsonian Center for Astrophysics | 2020 Oct 20
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 ... 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. ...This wide-field view of the center of our Milky Way galaxy shows, in visible light, the
vast array of stars contained within this small space. The stars closest to the center of
the galaxy, known as S-stars, are orbiting Sgr A*, a massive black hole. The orbits of
these stars are helping scientists to better understand the black hole and the nature of
our galaxy. This view was created from photographs in red and blue light and forming
part of the Digitized Sky Survey 2. The field of view is approximately 3.5° x 3.6°.
Credit: ESO and Digitized Sky Survey 2. Acknowledgment: D. De Martin and S. Guisard
Supermassive black holes like Sgr A*—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 ... "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 Sgr A* 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." ...
An Upper Limit on the Spin of Sgr A* Based on Stellar Orbits in Its Vicinity ~ Giacomo Fragione, Abraham Loeb
- Astrophysical Journal Letters 901(2):L32 (2020 Oct 01) DOI: 10.3847/2041-8213/abb9b4
- arXiv.org > astro-ph > arXiv:2008.11734 > 26 Aug 2020 (v1), 18 Sep 2020 (v2)