Event Horizon Telescope Reveals Curious Jet Structure from Black Hole

Scientists with the Event Horizon Telescope collaboration have peered deeply into the heart of the radio galaxy Centaurus A, revealing details of its long, powerful jets. The results appear July 19th in Nature Astronomy.

Cen A lies about 12 million light-years from us and hosts a supermassive black hole that’s actively swallowing gas and shooting out a narrow jet from top and bottom. The black hole isn’t a terribly voracious eater, compared with the leviathans that power quasars. But the galaxy is so close — and the jets tilted at just the right angle — that we can easily see the jets at radio wavelengths.

The team used observations taken at seven sites scattered across Earth’s western hemisphere, obtained during the EHT’s April 2017 campaign. Much as they did when reconstructing the now-iconic M87 black hole image, the team divided to conquer: A dozen individuals on the team worked independently to make 12 different images from the EHT data, using a range of methods. Then the team compared the results, weeded out the worst, and worked together to produce a final, reliable image.

The image looks peculiar at first glance. Hidden from view is the black hole; the team thinks it’s somewhere in the white circle. Given that marker, you’d be forgiven for thinking the black hole is shooting out four jets, but that’s not what’s happening. Instead, we’re looking at the edges of two jets, one pointing toward us (left) and the other counterjet pointing away (right).

Based on years of detailed simulations, astronomers think that the center of a black hole’s jet (its “spine”) is largely empty — it’s essentially a giant, corkscrew-like magnetic field with a few electrons and other particles zooming through, explains team member Michael Janssen (Max Planck Institute for Radio Astronomy, Germany). But sheathing this spine are an immense number of particles, which either blow off the black hole’s big fluffy gas disk or come from the surrounding galaxy. Because the jet doesn’t point directly at us, the glow from the fast-moving spine largely misses our line of sight, like a flashlight angled away from our eyes. But the ionized gas along the jet’s edges is moving around in a slower, less streamlined way, and its electrons are spiraling around the jet’s corkscrew. Those electrons emit photons at all sorts of angles, and the geometry works out such that we can see the edges quite well.

“What should be emphasized is how incredibly rare it is to have images on this scale, so close to the black hole,” says astronomer Eileen Meyer (University of Maryland, Baltimore County), who wasn’t involved with the study. “There are basically four or five.  So any new data on this scale is important.”

Especially interesting, both the team and Meyer point out, is that the edge-brightened jet structure also appears in the active galaxies M87, Markarian 501, and 3C84. These galaxies’ black holes all scarf down gas at fairly low rates, like Cen A’s. This trend may indicate there’s something universal going on in such systems.

Alas, the EHT can’t unmask Cen A’s black hole shadow: at 55 million solar masses, the black hole is too small to be resolved at these distances. Astronomers will need to move to higher radio frequencies — 100 times higher than what the EHT observes — and that means going to space. Linking two or more satellites together to work as an interferometer would potentially give astronomers the resolution they need, Janssen says.

Reference: M. Janssen et al. “Event Horizon Telescope observations of the jet launching and collimation in Centaurus A.” Nature Astronomy. Published online July 19, 2021.

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