A monster is spinning at the center of our galaxy and his portrait has finally been revealed.
Overnight, the international Event Horizon Telescope (EHT) crew revealed an image of superheated gas circulating and falling into Sagittarius A* or Sgr A*, the supermassive black hole at the core of the Milky Way.
It is the culmination of five years of simulations and data processing.
And while it may look a bit like a glazed donut, there’s more to the new image than meets the eye.
First, it tells us that the black hole is 4 million times the mass of the sun – a figure physicists suspectedbut has now been confirmed.
The black hole is also spinning, but it’s skewed — tilted a little towards us.
But despite this veritable goldmine of information about our galaxy’s black hole, there’s still a lot we have yet to discover.
What is so special about Sgr A*?
Well, for one, it is us supermassive black hole.
“It’s home,” said Jessica Dempsey, an Australian astrophysicist and member of the EHT team.
“That’s why this one is special to a lot of people. The hunt to understand what’s happening at the center of our galaxy is hundreds of years old.”
And while it may not be the largest black hole, its proximity to Sgr A* means it’s our best guess at understanding how it and its counterparts behave.
“As our instruments on the ground and in space improve our understanding, the Milky Way’s black hole will go a long way toward unpacking general relativity, and how that works with quantum mechanics,” said Dr. Dempsey, former deputy director of the East Asia Observatory in Hawaii.
If we know more about the massive heart of the Milky Way, we can provide clues about how our galaxy came to be.
“And maybe we can look for what we can learn from Sgr A*… in other galaxies,” she said.
An energy-inefficient giant
One of the biggest ongoing questions in black hole physics is how they collect, absorb, and expel material that orbits around them at nearly the speed of light in a process known as “accretion.”
This process is fundamental to the formation and growth of planets, stars and black holes of all sizes, throughout the universe.
Despite the bright spiral gas and dust in the image, Sgr A* doesn’t ‘eat’ as much matter as the team expected.
While some black holes can be remarkably efficient at converting gravitational energy into light, Sgr A* traps and holds almost all of this energy.
“It only converts one part in 1,000 into light,” said Dr. Johnson.
And unlike the giant black hole in the galaxy M87, an image of which was released in 2019, Sgr A* doesn’t shoot a huge beam of X-ray energy into space.
But it may have a weak beam, said Dr. Dempsey, based on heretofore inexplicable idiosyncrasies in how it rotates and accretes matter.
If there is indeed a jet, the EHT can’t see it yet, but research published late last year suggests a weak jet may be presentt.
As the EHT stared at the black hole, three X-ray telescopes also watched it. They saw X-rays — or bursts — from Sgr A*. Drawing a jet? Maybe.
Empty black holes to fill
James Miller-Jones, an astrophysicist at Curtin University and the International Center for Radio Astronomy Research, said measuring the polarized light thrown out by the black hole’s environment would tell us something about its magnetic field.
It’s something that EHT team reported on M87 last year†
“Sgr A* appears to have a strong, dynamically significant magnetic field, meaning it is a magnetic field strong enough to influence the movement of the plasma around the black hole,” said Professor Miller-Jones.
Alister Graham, an astrophysicist at the Swinburne University of Technology, hoped to find out how fast Sgr A*’s spin was.
“Black holes can orbit at significant fractions of the speed of light, but I felt… [the EHT team] couldn’t read this accurately.”
Another mystery yet to be solved is locating the launch site of plasma jets that inflated the galaxy’s colossal twin bubbles, he added.
So how are we going to answer these questions? First, let’s take a look at how astrophysicists managed to peek through a cosmic curtain of stars and gas at the black hole in our galaxy.
(Radio) lighting, (telescope) camera, action!
Over a handful of nights in April 2017, when the skies were clear, eight observatories from Antarctica to Europe set their sights simultaneously on the center of our galaxy, each tuned to pick up light at a wavelength of 1.3 millimeters.
These are radio waves – invisible to our eyes, but spewed out in abundance by the incredibly hot, turbulent gas that spins and falls into the black hole, producing the donut-like image.
Because the EHT observatories were separated by great distances, each telescope received the same radio signals from the center of the Milky Way at slightly different times.
Each data point of the radio signal was “stamped” at his telescope by an atomic clock so accurate it would lose just a second over the course of 100 million years.
When it came time to combine the data, these timestamps allowed physicists to synchronize the sequence of signals and generate a sharper image.
This coupled telescope technique, called Very Long Baseline Interferometry, essentially produces a telescope the size of the planet — and one with a resolution so high it could theoretically spot a ping-pong ball on the moon’s surface.
So how can it be improved? Funny you ask…
Did someone say more telescopes?
In the years since the first sightings in 2017, the EHT has refocused its radioactive eyes on Sgr A* – and more objects.
Since then, more observatories have joined the EHT network, which is already making a “very big” difference, said Dr. Dempsey.
More “eyes” means the EHT can capture more light, increasing its sensitivity and ability to spot weaker features.
“The more elements we bring in, the more sensitive we become and the more certain we can be whether we fit what we see … to the model,” said Dr. Dempsey.
“And the most important thing for Sgr A* is that we can make those snapshots faster.”
This means the team will eventually be able to capture images on the timescales they need to create a movie that captures dynamic features, such as the black hole’s rotation and the gases tumbling around it.
The EHT already has spatial resolution some 5,000 times better than the Hubble Space Telescope, giving the EHT a “fevering improvement” in its ability to spy on objects at great distances, Professor Graham said.
But to be able to discern finer details, we need more telescopes. But not on Earth.
“Having a radio telescope in space will provide more resolution, just like having one on the moon,” said Professor Graham.
That’s because the farther apart the network’s telescopes are, the better their spatial resolution.
There are plans to send a 10-meter-wide dish from a radio telescope about 1.5 million kilometers into space, where the gravity of the Earth and the sun will hold it in place.
When included in the Earth-based network, the telescope — dubbed the Millimetron Space Observatory — should give the EHT a 150-fold improvement in resolution.
The mission is led by the Russian Academy of Sciences and is so far scheduled to launch in 2030.
Looking in a different light
By tuning the EHT’s radio dishes to receive light of different wavelengths, astrophysicists will also get different representations of the black hole.
Detecting shorter wavelengths — less than a millimeter — should provide a sharper view through our galaxy’s disk, said Professor Miller-Jones.
Comparing the brightness of the black hole’s gaseous ring at different wavelengths — for example, if it appears brighter at one wavelength than the other — could reveal some of its physical processes.
“With the next generation [EHT] facility, it will be very exciting to test our models of the environment around the black hole, and what we understand about the processes of how gas flows around it,” said Professor Miller-Jones.
“That’s all going to be very, very interesting in the coming years.”
So there will no doubt be many more never-before-seen insights into some of the most mysterious phenomena in the universe — including our galaxy’s black hole.
“Personally, I love results that raise more questions than answers – and this [new image] is definitely one of those,” said Dr. Dempsey.
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