In 2019 astronomers lifted the veil on the monster black hole called Sagittarius A* (Sgr A*) at the heart of our Milky Way Galaxy. Using computer modelling, the scientists simulated the material inside the thick cloud of plasma, dust and gas surrounding Sgr A*. The results pointed to the possibility of a relativistic jet coming from the supermassive black hole with an inclination that is aligned with Earth’s viewing point.
An international team of astronomers recently used an Interferometric technique which combines many telescopes to form a virtual telescope the size of Earth to map out the exact properties of Sgr A*. Using telescopes including the Atacama Large Milimeter/submilimeter Array (ALMA) in northern Chile, an image was produced with a resolution that allowed us to peer through the fog surrounding the supermassive black hole.
Top left: simulation of Sgr A* at 86 GHz. Top right: simulation with added effects of scattering. Bottom right: scattered image from the observations, this is how we see Sgr A* on the sky. Bottom left: the unscattered image, after removing the effects of scattering in our line of sight, this is how Sgr A* really looks like. Credit: S. Issaoun, M. Mościbrodzka, Radboud University/ M. D. Johnson, CfA
To their surprise, they discovered that Sgr A*s emission is coming from an extremely narrow area of the sky — just one 300 milllionth of a degree. The emission also appeared to have a symmetrical shape. And, since black holes don’t emit detectable radiation on their own, the source is most likely one of two things.
Sgr A* –A disk of Infalling Gas or a Jet?
“This may indicate that the radio emission is produced in a disk of infalling gas rather than by a radio jet,” said astrophysicist Sara Issaoun of Radboud University in The Netherlands and a member of the EHT collaboration. “However, that would make Sgr A* an exception compared to other radio emitting black holes. The alternative could be that the radio jet is pointing almost at us.”
Since the 2019 observations, wrote Sera Markoff, American astrophysicist and professor of theoretical high energy astrophysics at the University of Amsterdam in an email to The Daily Galaxy, “Sgr A* does not, as far as we can tell, have relativistic jets at the moment, or at least nothing like those we see from our other Event Horizon Telescope (EHT) source galaxy M87. Those would be impossible to hide! It might have weak jets pointing towards Earth, but the point is that it wouldn’t matter since they are so weak they can’t even seem to make it outside of the Galactic center.”
The M87 Jet –100,000 Light Years Long at Radio Wavelengths
Compared to the ambiguous existence of the Sgr A* jet, the M87 jet (shown above), shoots out 5,000 light-years at optical wavelengths (100,000 light years at radio wavelengths), traveling at close to the cosmic speed limit. Using Chandra observations, researchers have seen that sections of the jet are moving at nearly the speed of light. When matter gets close enough to a black hole, it enters into a swirling pattern called an accretion disk. Some material from the inner part of the accretion disk falls onto the black hole and some of it is redirected away from the black hole in the form of narrow beams, or jets, of material along magnetic field lines. Because this infall process is irregular, the jets are made of clumps or knots that can sometimes be identified with Chandra and other telescopes.
EHT observed M87 over six days in April 2017, giving a snapshot of the black hole. The Chandra observations investigate ejected material within the jet that was launched from the black hole hundreds and thousands of years earlier. “It’s like the EHT is giving a close-up view of a rocket launcher,” said the CfA’s Paul Nulsen, “and Chandra is showing us the rockets in flight.”
Existence of Sgr A* Jets Debated
“Theoretically speaking,” Markoff added, “Sgr A* has all the conditions to launch weak jets, so many of us suspect they are present, just difficult to detect because the Galactic center is a very complicated region with lots of confusing features that could hide weak jets.”
“Since we cannot prove the existence of jets there has been controversy for years,” Markoff wrote to The Daily Galaxy, “The radio spectrum looks very much like that of other supermassive black holes that are weakly accreting like those in nearby galaxies such as the weak jet in M81, which is almost a twin of our Galactic center but a bit higher power. Also the variability pattern moves from high to low frequency, which is the opposite of what you would expect for infalling gas, because high frequency light comes from more compact regions. Thus stuff moving outwards from compact regions near the black hole, as in a jet, would show high to low frequency ‘waves’ of variability, and we do see that.
EHT and Radio VLBI Will Help Continue Lifting the Veil
“I do think EHT and radio VLBI in general will help us sort this out,” Markoff concludes, “but I would not bet that our single epoch of observations from 2017 will be enough. Most likely we will need several years of observations to build up enough certainty, as well as reliable ‘movies’ of what’s going on in the source. Not only to resolve the question of if there is a jet, but which direction is it pointing, and does that direction line up with the black hole spin (it doesn’t have to!)?”
Markoff is a member of the Event Horizon Telescope team that produced the first ever image of the massive now-iconic black hole at the center of M87 described by scientists on April 10, 2019 at the press conference in Brussels where the photograph was revealed as the “Gates of Hell” and the “End of Spacetime”. An image described as “fathomless dark creations of the Universe”— equal to the famous “Earthrise” photo taken by Apollo 8 astronaut Bill Anders in December 1968.
“While it’s possible that Sgr A* drives a relativistic jet,” says Daryl Haggard, Associate Professor of Physics at McGill University in the McGill Space Institute told The Daily Galaxy, “but if it’s there, it’s nowhere near as powerful as the one we have beautifully imaged in M87. This doesn’t necessarily mean that Sgr A* is an exception,” Haggard adds, “not all supermassive black holes drive powerful jets, but it is a bit of a puzzle – theoretically we think a jet should be there but we haven’t convincingly detected one yet. It’s definitely the case that a substantial portion of the radio and sub-mm emission from Sgr A* comes from the hot plasma swirling around the black hole. Stay tuned though, observing Sgr A* never gets boring and we’ll have more rich data to share soon!”
Haggard leads multi-wavelength, time domain studies of growing supermassive black holes, including Sagittarius A* and M87. She was a member of the Event Horizon Telescope collaboration in reporting the first direct image of the M87 black hole’s shadow in 2019 and the EHT team receiving the 2020 Breakthrough Prize in Fundamental Physics.
“A One-way Door Out of Our universe,” –How the Scientists Described the 2019 Image
We gave humanity its first view of a black hole — “a one-way door out of our universe,” said EHT project director Sheperd S. Doeleman of the Center for Astrophysics, of the image of the massive black hole at the center of elliptical galaxy M87. “This is a landmark in astronomy, an unprecedented scientific feat accomplished by a team of more than 200 researchers.”
The M87 black hole really is a monster, observed Ellie Mae O’Hagan for The Guardian. “Everything unfortunate enough to get too close to it falls in and never emerges again, including light itself. It’s the point at which every physical law of the known universe collapses. Perhaps it is the closest thing there is to hell: it is an abyss, a moment of oblivion.”
Astrophysicist Janna Levin author of “Black Hole Blues” with Columbia University noted for The Guardian that we are actually seeing the black hole as it was 55 million years ago, because it’s so far away the light takes that long to reach us. “Over those eons, we emerged on Earth along with our myths, differentiated cultures, ideologies, languages and varied beliefs,” she says. “Looking at M87, I am reminded that scientific discoveries transcend those differences.”
The Daily Galaxy, Jackie Faherty, astrophysicist, Senior Scientist with AMNH via Radboud University and Sera Markoff, University of Amsterdam and Daryl Haggard, McGill University. Jackie was formerly a NASA Hubble Fellow at the Carnegie Institution for Science.
Image at the top of the page: Shutterstock License
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