There’s a monster swirling around the center of our galaxy, and its portrait has finally been revealed.
Overnight, the international crew of the Event Horizon Telescope (EHT) revealed an image of superheated gas flowing and falling into Sagittarius A* or Sgr A*, the supermassive black hole at the heart 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, the new image has more to offer than meets the eye.
On the one hand, it tells us that the black hole is 4 million times the mass of the Sun – a figure that physicists suspected, but has now been confirmed.
The black hole is also rotating, but it is oblique – slightly tilted in front of us.
But despite this veritable wealth of information about our galaxy’s black hole, there is still much to discover.
What’s so special about Sgr A*?
Well, on the one hand it is our a supermassive black hole.
“This is my 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 figure out what’s going on at the center of our galaxy goes back hundreds of years.”
And while it may not be the biggest black hole, Sgr A*’s proximity means it’s our best bet for 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 in unpacking general relativity and how it works with quantum mechanics,” said Dr. Dempsey, former deputy director of the ‘East -Asian Observatory in Hawaii.
Knowing more about the heavy heart of the Milky Way may give clues to the formation of our galaxy.
“And maybe what we can learn from Sgr A*, we can start looking…in other galaxies,” she said.
An energy giant
One of the biggest ongoing questions in black hole physics is exactly how they collect, ingest, and expel materials that orbit them at near-lightspeed in a process called “accretion.”
This process is fundamental to the formation and growth of planets, stars and black holes of all sizes throughout the universe.
Despite the glowing spiraling gas and dust in the image, Sgr A* wasn’t “eating” as much material as the team had expected.
While some black holes can be remarkably efficient at converting gravitational energy into light, Sgr A* traps and clings to almost all of that energy.
“It only converts 1 in 1,000 parts to light,” Dr. Johnson said.
And unlike the gargantuan black hole in the galaxy M87, an image of which was released in 2019, Sgr A* isn’t sending a huge burst of X-ray energy out into space.
But it might have a weak jet, Dr. Dempsey said, based on as yet unexplained peculiarities in the way it spins and accretes matter.
If a jet is indeed there, the EHT cannot see it yet, but research published late last year suggests that a weak jet may be present.
While the EHT watched the black hole, three X-ray telescopes also watched it. They spotted X-ray flares – or explosions – of Sgr A*. Signs of a jet? Perhaps.
Draft black holes to be filled
James Miller-Jones, an astrophysicist at Curtin University and the International Center for Radio Astronomy Research, said measuring polarized light emitted from the black hole’s surroundings would tell us about its magnetic field.
This is something the EHT team reported last year about M87.
“Sgr A* appears to have a strong and dynamically significant magnetic field, meaning it’s a strong enough magnetic field to affect the motion of the plasma around the black hole,” Professor Miller-Jones said.
Alister Graham, an astrophysicist at Swinburne University of Technology, hoped to find out how fast Sgr A* was spinning.
“Black holes can spin at significant fractions of the speed of light, but I felt [the EHT team] was unable to get an accurate reading on this.”
Another unsolved mystery involves locating the launch site of the plasma jets that blew up the Milky Way’s colossal twin bubbles, he added.
So how are we going to answer these questions? First, let’s see how astrophysicists managed to peek through a cosmic curtain of stars and gas to our galaxy’s black hole.
Lights (radio), camera (telescope), action!
On a handful of nights in April 2017, when skies were clear, eight observatories from Antarctica to Europe simultaneously focused their gaze on the center of our galaxy, each tuned to record light with a length of wave of 1.3 millimeters.
These are radio waves – invisible to our eyes, but spit out in abundance by the incredibly hot and turbulent gas swirling and falling into the black hole, which produces the doughnut-shaped 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 precise that over the course of 100 million years it would only lose a second.
When it was time to combine the data, these timestamps allowed physicists to synchronize the multitude of signals and generate a cleaner picture.
This linked telescope technique, called very long baseline interferometry, essentially produces a planet-sized telescope – and one with such high resolution that it could, in theory, spot a ping-pong ball on the surface of the moon.
So how can it be improved? It’s funny that you ask…
Did someone say more telescopes?
The EHT trained its radio-ready eyes again on Sgr A* – and even more objects – in the years since its first sightings in 2017.
Other observatories have since joined the EHT network, which is already making a “really huge” difference, Dr Dempsey said.
More “eyes” means the EHT can collect 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 that we fit what we see…to the model,” Dr. Dempsey said.
“And the most critical part for Sgr A* is that we can do those snapshots faster.”
This means the team will eventually be able to take images on the timescales they need to produce a movie that captures dynamic features such as the rotation of the black hole and the gases collapsing around it.
Already, the EHT has a spatial resolution around 5,000 times better than the Hubble Space Telescope, giving the EHT a “tremendous improvement” in the ability to spy on objects at great distances, Prof Graham said. .
But to do finer details, we will need more telescopes. Not on Earth, however.
“Having a radio telescope in space will provide additional resolution gains, just like having one on the Moon,” Professor Graham said.
Indeed, the further away the telescopes in the network are, the better their spatial resolution.
Plans are underway to send a 10-metre-wide radio telescope dish about 1.5 million kilometers into space, where the gravitational tug of Earth and the Sun will hold it in place.
When integrated into the terrestrial array, the telescope – called the Millimeter Space Observatory – is expected to give the EHT a 150-fold improvement in resolution.
The mission is led by the Russian Academy of Sciences and is currently scheduled for launch in 2030.
See in a different light
Tuning the EHT’s radio dishes to pick up light of different wavelengths will also give astrophysicists different representations of the black hole.
Detecting shorter wavelengths – less than a millimeter – should provide a sharper view across our galaxy’s disk, Professor Miller-Jones said.
Comparing the brightness of the black hole’s gaseous ring at different wavelengths – for example, whether it appears brighter in one wavelength than the other – could reveal some of its physical processes.
“With the next generation [EHT] installation, it will be very exciting to test our models of the environment around the black hole and what we understand of the gas flow processes around it,” said Professor Miller-Jones.
“All of this will be very, very interesting in the years to come.”
So there will no doubt be plenty more never-before-seen information about some of the universe’s most mysterious phenomena, including our galaxy’s black hole.
“Personally, I like results that open up more questions than answers – and that [new image] is definitely one of them,” Dr. Dempsey said.
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