What will we actually see? When it’s completed, the picture of the Milky Way’s supermassive black hole, Sagittarius A* (Sgr A*), is an image sure to equal the famous “Earthrise” photo taken by Apollo 8 astronaut Bill Anders in December 1968. The obvious target for the Event Horizon Telescope (EHT), the team hopes to get imagery of our supermassive black hole soon, said Shep Doeleman, Director, Event Horizon Telescope, following the first ever image of Galaxy M87’s gargantuan black hole (above).
The researchers looked at M87 below, first, because it’s an enormous elliptical galaxy 55 million light-years away that harbors a mind-boggling supermassive black hole somewhere between 3.5 billion and 7.2 billion times the mass of the sun. It’s a bit easier to resolve than Sagittarius A* because it’s less variable over short timescales, Doeleman explained. For comparison, Sgr A* is estimated to be about a thousand times less massive, with about 4 million times the mass of the sun.
M87 is an enormous elliptical galaxy 55 million light-years away that harbors a mind-boggling supermassive black hole somewhere between 3.5 billion and 7.2 billion times the mass of the sun. “At the small end of that range, M87 would be an impossible target for EHT, Doeleman observed. At the high end, it is possibly suitable. So M87 became a secondary target in the pursuit of Sagittarius A*.
Polarization of light — Reveals the Physics Behind the Image
This is the first time astronomers have been able to measure polarization, a signature of magnetic fields, this close to the edge of a black hole. The observations are key to explaining how the M87 galaxy is able to launch energetic jets from its core”.
“We are now seeing the next crucial piece of evidence to understand how magnetic fields behave around black holes, and how activity in this very compact region of space can drive powerful jets that extend far beyond the galaxy,” says Monika Mościbrodzka, Coordinator of the EHT Polarimetry Working Group and Assistant Professor at Radboud University in the Netherlands.
A Major Milestone
This work is a major milestone: the polarization of light carries information that allows us to better understand the physics behind the image we saw in April 2019
The EHT collaboration delved deeper into the data on the supermassive object at the heart of the M87 galaxy collected in 2017. They discovered that a significant fraction of the light around the M87 black hole is polarized.
“The polarization of light carries information that allows us to better understand the physics behind the image we saw in April 2019, which was not possible before,” explains Iván Martí-Vidal, also Coordinator of the EHT Polarimetry Working Group and GenT Distinguished Researcher at the University of Valencia, Spain. He adds that “unveiling this new polarized-light image required years of work due to the complex techniques involved in obtaining and analyzing the data.”
Milky Way’s Sagittarius A* —The Last Photon Orbit
What we’ll see when the EHT actually sees Sagittarius A* is an area slightly outside the event horizon itself — a region defined by the location closest to the black hole where a beam of light could orbit on a circle, known as the “last photon orbit.”
“Another cool aspect of the M87 EHT image was the degree of polarization in the accretion disk, revealing the structure of magnetic fields in the final orbits around the SMBH,” says Maxwell Moe, dailygalaxxy.com editor and NASA Einstein fellow at the U of Arizona. “We may also see polarized light in an EHT image of Sag A*.
Were you to float there, says astrophysicist Janna Levin, professor of physics and astronomy at Barnard College of Columbia University and author of Black Hole Blues, “you could see light reflected off the back of your head after completing a round trip. Or, if you turned around quickly enough, you might see your own face. Closer than that, all the light falls in.”
A Dark Shadow the Size of Our Solar System
The M87 EHT image is unmistakable — a dark shadow the size of our solar system, writes Levin, enveloped by a bright, beautiful blob.
The first ever photograph above of the black hole shadow in the very center of M87 galaxy was published in April 2019 by the Event Horizon Telescope. This figure shows the polarized image of M87 black hole, published on March 24, 2021.
The Anthropological Impact
“While the scientific implications will take time to unpack”, Levin says, “some of the anthropological impact feels immediate. The light EHT collected from M87 headed our way 55 million years ago. Over those eons, we emerged on Earth along with our myths, differentiated cultures, ideologies, languages and varied beliefs. Looking at M87, I am reminded that scientific discoveries transcend those differences. We are all under the same sky, all of us bound to this pale blue dot, floating in the sparse local territory of our solar system’s celestial bodies, under the warmth of our yellow sun, in a sparse sea of stars, in orbit around a supermassive black hole at the center of our luminous galaxy.”
Image Credit top of page: M87 Black Hole EHT, Victor Tangerman