New black hole images contain clues to cosmic mysteries

A massive effort to view a black hole in multiple ways is helping scientists untangle what happens when gravity is pushed to extremes.

Published 19 Apr 2021, 10:19 BST
black hole
A view of the M87 supermassive black hole in polarised light: The Event Horizon Telescope (EHT) collaboration, who produced the first ever image of a black hole released in 2019, has today a new view of the massive object at the centre of the Messier 87 (M87) galaxy: how it looks in polarised light. This is the first time astronomers have been able to measure polarisation, a signature of magnetic fields, this close to the edge of a black hole. This image shows the polarised view of the black hole in M87. The lines mark the orientation of polarisation, which is related to the magnetic field around the shadow of the black hole.
Photograph by Image by EHT Collaboration

In the heart of a gargantuan galaxy 55 million light-years away, a black hole with the heft of 6.5 billion suns is hurling a fountain of matter into the cosmos at near light-speed. Using an array called the Event Horizon Telescope (EHT), scientists harnessed radio waves to capture a mugshot of that black hole, offering our first-ever look at the extreme environment near its edge in 2019

Two years later, the international team that delivered the astounding image, along with additional partners, has published the results of a 2017 observing campaign that simultaneously scrutinised the host galaxy, Messier 87, in multiple wavelengths.

The report, which appears in The Astrophysical Journal, includes data from 19 Earth- and space-based observatories, and is authored by more than 750 scientists. It describes a more complete view of the supermassive black hole and its colossal jet, letting scientists take a good look at how magnetic fields, particles, gravity, and radiation interact within the vicinity of a supermassive black hole on multiple scales.

“This is the kitchen sink of physics, right? Everything is in there,” says McGill University’s Daryl Haggard, who helped coordinate the multiwavelength observations. “We’re really starting to see orbits, we’re seeing right down next to the black hole and probing this exotic environment.”

“I think this is one of the papers that really connects EHT to the rest of the community—it’s a taste of what the facility really is meant to do,” adds team member Sera Markoff of the University of Amsterdam. “I feel like this is at the beginning of everything.”

Now the EHT team is in the midst of a crucial 12-day observing run—the first they’ve been able to do since 2018, due to technical problems and the coronavirus pandemic. This time around, the collaboration has added three new telescopes to its retinue of observatories, including a facility in Greenland, and it is again scanning the sky in wavelengths spanning the electromagnetic spectrum—as long as the weather cooperates. 

“You need to have really good weather at all sites,” says Radboud University’s Monika Moscibrodzka. “And the more sites you have, the lower the probability of good weather at each of them.”

A cosmic cruller

Black holes have been among the more intriguing, compelling astronomical phenomena for more than a century, capturing our imaginations with their extreme physics and the fact that what goes in never comes back out. But these cosmic sinkholes have only recently come into focus, thanks to the EHT image, as well as Nobel-prize winning studies of objects zipping around the supermassive black hole at the core of the Milky Way and a wealth of information gleaned from watching as black holes smash into one another.

“In the last few years, we went from black holes being science fiction to black holes being reality,” says Marta Volonteri of the Institut d’Astrophysique de Paris.

The Event Horizon Telescope actually comprises multiple radio telescopes scattered around the globe, from Greenland to the South Pole, that act together as an Earth-size observatory. Making these images of M87’s supermassive black hole requires combining an enormous amount of data—so much data that the team can’t digitally transfer it and instead has to drop hard drives in the mail. 

When the team released their first image in April 2019, scientists were stunned because the object looked almost exactly as predicted by a century-old theory.

M87’s image offered a chance to test Einstein’s 1915 theory of general relativity, which posits that what we perceive as gravity emerges when matter curves the fabric of space-time. The environment around M87’s heart is intense—a hot mess of extreme gravity, magnetic fields, and particles—which makes it one of the best places in the universe to challenge general relativity.

“Everybody is always trying to break these theories, because we learn so much when we find a chink in the armour,” says Haggard. “We love breaking models. But we haven’t successfully broken general relativity yet.”

While general relativity prevailed again with M87, the EHT image quickly worked its way into the public consciousness. The brainy comic strip XKCD featured the team multiple times, and superimposed the solar system atop the black hole’s maw to show its scale. Others compared its glowing ring to the Eye of Sauron from The Lord of the Rings movies. But the most energetic debate erupted over its resemblance to breakfast food.

“Is it more like a bagel or a donut?” Volonteri asks.

An update to that original image, assembled by Moscibrodzka and her colleagues, settled the argument last month: the black hole looks like a cruller, or a grooved donut. In the newer image, signatures of the black hole’s magnetic field are layered atop the original glowing ring, revealing a smooth, organised pattern that wraps around the hulking object. Moscibrodzka and her colleagues studied charged particles that trace magnetic field lines to provide a more detailed look at the extreme physical conditions surrounding the black hole.

Colouring in a place that light never leaves

Now, as reported in the new study, multiwavelength observations are further colouring in that tasty image.

Scientists are hoping these combined observations will help reveal the physics powering the mammoth jet of particles erupting from M87’s core. The jet spans thousands of light-years, stretching across the galaxy, and is somehow launched from the pool of blistering plasma, twisted magnetic fields, and other matter whirling around the black hole.

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Scientists suspect that such jets could be responsible for a population of extremely high-energy cosmic particles that make their way into our neighbourhood, where they’re known as cosmic rays. Although the sun blows a protective bubble around much of the solar system, energetic particles can still slip through, and some of the ones that slam into Earth’s atmosphere are travelling at such immense speeds that they cannot have originated from within the Milky Way.

“One of the primary questions we’re trying to investigate is where are the high-energy particles coming from,” Markoff says. “How are these jets launched, what’s inside of them, and how are high-energy cosmic rays—which seem to be coming from black hole jets—accelerated? You cannot answer these questions with EHT alone.”

With the new observations, scientists can better understand the jet—which emits light at every wavelength, from radio waves to gamma rays—and see if it is, in fact, slinging matter into space at a speed that Earth’s largest particle accelerators could never equal.

As well, a better picture of the jet’s anatomy could reveal some still-mysterious properties about M87’s black hole, such as how fast it is spinning, and in which orientation. Those measurements will offer clues about how the supermassive black hole grew, and whether in the last billion years it has gained mass primarily from collisions with other supermassive black holes, or from feasting on surrounding gas.

“In some sense, the spin has a better memory of how black holes grow in mass than actually measuring the mass,” Volonteri says.

On the EHT’s horizon

As this week’s observing campaign unfolds, scientists are again aiming their telescopes at M87 to see how it might have changed. The black hole was in a quiescent, slumbering state during the 2017 observing campaign, which let the team see right into its core. Now, “we are very curious to see how M87 will evolve on longer timescales—we’re curious about what we’re going to get this time,” Moscibrodzka says.

The EHT team is also taking a peek at the supermassive black hole nearest to home: Sagittarius A*, or SgrA*, which is parked in the heart of the Milky Way. With a mass equal to roughly four million suns, SgrA* is less hefty than the bruiser in M87, but it’s also much, much closer to Earth and the EHT, at only 25,600 light-years away.

However, our resident supermassive black hole is also more temperamental. It frequently belches and flares as it gobbles up material, sometimes having outbursts over the course of a single evening. Those fluctuations in activity are one of the reasons why it’s taken longer to put together an image.

“From an observational perspective, that introduces a whole lot of challenges,” Haggard says. “How do you make a stable image of something that varies all of the time?”

It’s a tough challenge, but an image of SgrA* is on the horizon—and soon, with heaps of observations in hand, we’ll be many steps closer to understanding the churning enigmas that lurk in the hearts of galaxies, and create some of the most extreme phenomena in the observable universe.

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