Unlike the now iconic gargantuan black hole the size of our solar system at the center of nearby elliptical galaxy, M87, with its massive jet that blasts out at near the speed of light, the Milky Way’s central black hole, SgrA*, which until May, 2019 when it suddenly brightened, appeared like a massive, dormant volcano, a sleeping monster, a slumbering region of spacetime where gravity is so strong that “what goes into them does not come out”– seems, according to new research, not to have a jet and diminished spin, “proving to be stranger than science fiction.”
The Theoretical Foundations
UC Berkeley’s Reinhard Genzel and UCLA’s Andrea Ghez were awarded the 2020 Noble Prize for physics based on ground-breaking research –“Theoretical Foundation for Black Holes and the Supermassive Object at the Galactic Center”–which demonstrated that this supermassive compact object at the center of our galaxy, SgrA*, is a black hole. “Each lead a group of astronomers that, since the early 1990s, has focused on a region called Sagittarius A* at the center of the Milky Way galaxy. The orbits of the brightest stars closest to the middle of the Milky Way have been mapped with increasing precision. The measurements of these two groups agree, with both finding an extremely heavy, invisible object that pulls on the jumble of stars, causing them to rush around at dizzying speeds. Around four million solar masses are packed together in a region no larger than our solar system.”
Near Speed-of-Light Jet
This near-speed-of -light of M87s jet is called Superluminal motion, which was captured by the Chandra X-Ray Observatory images between 2012 and 2017 of ejected material within the jet that was launched from the black hole hundreds and thousands of years earlier, occurs when objects are traveling close to the speed of light along a direction that is close to our line of sight. The jet travels almost as quickly towards us as the light it generates, giving the illusion that the jet’s motion is much more rapid than the speed of light. In the case of M87*, the jet is pointing close to our direction, resulting in these exotic apparent speeds.
Two Critical Numbers –Mass and Spin
Supermassive black holes like SgrA* are characterized by just two numbers: mass and spin, that have a critical influence on the formation and evolution of galaxies, according to the new study by the Harvard Center for Astrophysics (CfA), and the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) at Northwestern University. According to 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.”
Genzel and Ghez monitored the motion of stars around SgrA*” 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.
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 Reveal
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.”
Jets as Giant Flywheels
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.”
Source: “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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