Black-hole image sheds light on Milky Way mysteries

Black-hole image sheds light on Milky Way mysteries

The first image of our Galaxy’s supermassive black hole, released earlier this month, has already begun to explain some enduring mysteries about the heart of the Milky Way.

The wealth of new information about the black hole, called Sagittarius A*, joins many other lines of evidence that are now painting a detailed picture of the Galactic Centre. Taken together, the results suggest that Sagittarius A* is sucking in matter at a slow pace, making it unusually dim compared with the central black holes of other galaxies. The observations also hint that Sagittarius A* could have been spectacularly active only a few million years ago. Meanwhile, the latest data are raising fresh questions about some of the largest structures seen in and around the Milky Way.

The image, released by the Event Horizon Telescope (EHT) collaboration on 12 May, was the highlight of a set of ten papers in a special issue of Astrophysical Journal Letters1. But the underlying data, gathered in 2017, contain much more information that scientists are still combing through, says EHT member Sera Markoff, a theoretical astrophysicist at the University of Amsterdam. “This is like heaven” for astrophysicists, she says.

The image shows a glowing ring of radio emissions surrounding a dark shadow. This shadow lies just beyond the black hole’s event horizon — the intangible sphere that marks a point of no return for anything that crosses it. Detailed analysis of the EHT data has now confirmed many aspects of theoretical and computer models describing how the glowing ring is produced.

As matter spirals into the black hole at nearly the speed of light, it forms an ‘accretion disk’ that emits radiation across the electromagnetic spectrum, including radio waves that the EHT’s telescopes can detect. Their data show that the accretion disk is shaped more like a puffed-up doughnut than a flat pancake. This fattened shape means that the disk supplies the black hole with scraps of matter at a leisurely pace, which makes it relatively dim compared with other, greedier black holes.

This is the first image of Sgr A, the supermassive black hole at the centre of our galaxy.

The EHT collaboration released this image of the black hole Sagittarius A* earlier this month.Credit: EHT Collaboration

Although the shape of the accretion disk met expectations, many astrophysicists were surprised that the EHT’s data showed the disk ‘face on’. This means its axis of rotation is angled at less than 50° from our line of sight from Earth.

Some scientists had expected that the disk’s axis of rotation would instead point vertically, showing the accretion disk ‘edge on’ from Earth’s point of view. This orientation would arise from the interplay of three separate rotations: the stately turn of the Galaxy’s spiral arms, the infalling matter supplying the accretion disk, and the rapidly spinning black hole itself.

Sagittarius A* probably formed from the merger of two black holes, when a pair of galaxies combined to formed the Milky Way. Initially, the spin of the new black hole could have pointed in any direction. But as it grew by feeding on dust and gas, the momentum of infalling matter would have slowly aligned the black hole’s spin with that of the Galaxy, says Priya Natarajan, an astrophysicist at Yale University in New Haven, Connecticut. Because the Milky Way hasn’t had a merger in at least one billion years, all three rotations should have lined up by now.

However, the EHT’s preliminary results have almost certainly ruled out a vertical spin axis for the accretion disk, and perhaps also for the black hole itself. This matches observations made in 2018 by the Very Large Telescope (VLT), a facility on the mountain Cerro Paranal in Chile, which saw flares from matter orbiting very close to the black hole’s event horizon in a clockwise direction, just where the EHT saw its ring. “You could actually superimpose those two images,” says Stefan Gillessen, a radio astronomer at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.

Gillessen and his collaborators conducted the study using the GRAVITY instrument, which collects infrared light from the VLT’s four 8-metre dishes to achieve a resolution comparable to that of a single 130-metre-wide telescope. Like the EHT, GRAVITY found that the accretion disk has a face-on orientation, with its axis of rotation angled 20–30° from our line of sight.

This face-on orientation is also consistent with decades of observations of the structure of the Milky Way’s central region, says Jason Dexter, a theoretical astrophysicist at the University of Colorado Boulder, who is a member of both the GRAVITY and the EHT collaborations. The black hole’s accretion disk is supplied by matter flowing from stars that orbit Sagittarius A* in a disk about 0.3 parsecs (one light year) across, he says. So the orientation of the accretion disk should match the disk of stars, rather than the larger-scale structure of the Galaxy, Dexter says. “There is no problem there — and maybe we should have expected it.”

The EHT’s 2017 data cannot yet confirm the clockwise rotation of the accretion disk seen by GRAVITY, says Charles Gammie, a member of the EHT collaboration at the University of Illinois at Urbana–Champaign. But the team has been gathering more data, and it could soon answer that question. “The new observations from 2022 may have enough information, especially if we can make a movie and see structures rotating,” Gammie says.

Spiral streams

Zooming out from the centre of the Galaxy, astronomers have previously mapped several other larger structures up to a few parsecs across. These include a ‘mini-spiral’ made of streams of gas that are reminiscent of the Milky Way’s spiral arms, but 10,000 times smaller. There does not seem to be much matter falling inwards from the spiral right now, but in the past it could have fed the black hole during periods of much more intense activity.

Interestingly, this spiral does not align with the disk of stars around Sagittarius A*, nor with its accretion disk or with the Galaxy itself. “The very centre of the Galaxy doesn’t have to align with the plane of the Galaxy,” says Markoff. “You don’t necessarily expect the stuff that’s happening very close to the black hole to know anything about the galactic plane.”

Models, such as Natarajan’s, that predict a gradual alignment of the black hole’s spin might apply only to galaxies that supply a steady stream of matter to the black hole over a long time, says Andrew King, an astrophysicist at the University of Leicester, UK. That doesn’t seem to be the case for the Milky Way, nor for many other galaxies that seem to contain misaligned central black holes. “The reason must be that the gas feeding the black hole is not directed in an orderly way, but comes in separate episodes whose directions are arranged completely randomly compared with the black-hole spin axis,” King says.

This kind of chaotic feeding could keep the black hole spinning at a fairly slow rate, which would allow it to accrete enough matter to grow rapidly. That could help to explain how some black holes grew so big, so quickly: some were already billions of times as massive as the Sun when the Universe was one-tenth of its current age.

Blowing bubbles

Although all of these pieces of evidence seem to agree on the orientation of Sagittarius A*, there are still big questions about a possible connection between the black hole and other huge features seen around the Galaxy’s centre.

In 2010, astronomers using NASA’s Fermi Gamma-ray Space Telescope mapped two enormous lobes of gas extending directly above and below the central region of the Galaxy, each 7,700 parsecs long. These lobes glow in X-rays, and have become known as the Fermi bubbles. And in 2020, the eROSITA X-ray telescope aboard a German–Russian probe detected even larger bubbles in the same region of space.

A composite Fermi–eROSITA image comparing the morphology of the γ-ray and X-ray bubbles.

Composite image showing Fermi bubbles (red) and the bubbles detected by eROSITA (blue).Credit: P. Predehl et al./Nature

The observations suggest that these bubbles are the afterglow from shock waves that jutted out of the Galactic Centre in the past 20 million years or so. A plausible source for such a shock wave could be a burst of star-forming activity, leading to a large number of stellar explosions called supernovae. But another major suspect is a period of intense feeding from Sagittarius A*.

Researchers have also found glowing columns of gas extending more than 150 parsecs from the Galactic Centre, which might indicate that Sagittarius A* created the Fermi bubbles. “Like a chimney that’s still hot from smoke and heat that just went through it, these chimneys could be a relic of the outflow that inflated the Fermi and eROSITA bubbles,” says astrophysicist Gabriele Ponti, a colleague of Gillessen’s at the Max Planck Institute in Garching.

But the bubbles seem to be aligned vertically with the Milky Way’s axis, so it’s unclear how they could have originated from a black hole that is tilted in a different direction. One possibility is that the bubbles are the end result of many separate periods of intense feeding, each spewing out matter in a different direction. “What the EHT has shown was a snapshot. The Fermi bubbles reveal activity over very long timescales,” says Simona Murgia, an astronomer at the University of California, Irvine, who works on the Fermi mission.

An X-ray space telescope called Athena, planned for launch by the European Space Agency in the mid-2030s, could help to settle the question by mapping the motion of the gas in the Fermi bubbles, Ponti says.

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