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The sky is full of weird X-shaped galaxies. Here's why.

The 'double boomerang' of an x-shaped radio galaxy.
The 'double boomerang' of an x-shaped radio galaxy. These big blue jets are about 100 times longer than the entire Milky Way.
(Image: © NRAO/AUI/NSF; SARAO; DES)

Spied through a normal telescope, the galaxy PKS 2014−55 is an unremarkable smudge of bright light. But look again in radio wavelengths, and you'll see that the galaxy is hiding a gargantuan, glowing treasure at its center — and X marks the spot.

PKS 2014−55 is an X-shaped radio galaxy (XRG), an unusual type of galaxy that looks like an enormous X in the night sky when imaged in radio wavelengths. The long arms of the X — each one about 100 times longer than the Milky Way — are actually a blazing-fast soup of particles and magnetic fields, blasted out of the galaxy's central black hole and traveling for millions of light-years into space, far beyond the galaxy's edge.

Big jets of radio energy are common in galaxies with hungry black holes at their centers (even the Milky Way has two "bubbles" of radio energy around its gut). However, most of those jets come in orderly pairs that appear to form a straight line or a round bulge when seen from far away. According to William Cotton, an astronomer at the National Radio Astronomy Observatory (NRAO) in Virginia who studies XRGs, fewer than 10% of known cosmic radio sources take on a distinct X shape like this one.

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"You see four things poking out of this galaxy," Cotton, lead author of a new study on the galaxy, told Live Science, "and the question is, how did it get that way?"

Now, Cotton and his colleagues may have an answer. In a new study posted May 7 on the pre-print server arXiv and accepted for publication in the journal Monthly Notices of the Royal Astronomical Society, researchers with the NRAO and South African Radio Astronomy Observatory (SARAO) used the massive MeerKAT radio telescope in South Africa's Karoo desert to capture the most detailed image of an XRG ever. The image, shown above, reveals that the strange X bursting out of PKS 2014−55's center apparently isn't an X at all.

"It's actually a 'double boomerang' shape," Cotton said. "That means something in the galaxy is diverting the flow into these secondary wings."

This image shows the two youngest radio jets (white dots) blasting out of the galaxy's central black hole, while two older jets continue leaking into space after tens of millions of years. (Image credit: UP; NRAO/AUI/NSF; SARAO; DES)

According to Cotton and the new study, the galaxy's strange shape can be explained by a theory known as the "hydrodynamical backflow model." Here's what's happening, in a nutshell: First, the galaxy's central black hole gobbles up matter for millions of years, until it experiences a bout of cosmic indigestion. The black hole belches twin jets of matter into space, each traveling in opposite directions at incredible speed.

Eventually (tens of thousands of years later), those jets blast through the galaxy's gassy halo, traveling onward into intergalactic space. Pressure slowly builds up in the jets as they travel farther and farther out of the galaxy, ultimately forcing some material in each jet to flip around and flow back toward the center again. This phenomenon is known as "backflow."

Backflow is common in active galaxies, Cotton said, but usually all that returning material bulges up in the middle of the galaxy, rather than bouncing off to the side. In PKS 2014−55, the galaxy's hot halo of dust and gas is angled in such a way that the backflow is actually "deflected" back out of the galaxy, giving each jet a boomerang-like appearance.

To Cotton and his colleagues, this long cosmic history was evident simply by looking at the contours of the jet flow in the MeerKAT image — "We looked at the image for about 10 seconds and just said, 'yeah, that's it,'" Cotton said. But as the team analyzed the brightness of the jets, further details emerged.

According to Cotton, this image shows not one, but three separate black hole burstss separated by tens of millions of years. The two white dots near the picture's center show the most recent event, with twin lobes of energy just beginning to expand out of the black hole and into the galaxy. Beyond these, the two long blue lobes are the decaying remnants of two jets that erupted from the black hole around 10 million years ago, Cotton said.

"And, if you look around the edges of the jets, there's what we call a 'cocoon' — the faint remnant of an even earlier outburst," he added. "That's something on the order of 100 million years old."

The fact that all three generations of jets appear to follow the same boomerang pattern suggests that hydrodynamical backflow explains this XRG's shape, rather than some other phenomenon (such as the black hole changing direction between one jet outburst and the next).

The same model may not explain every X-shaped galaxy in the universe — each one requires its own analysis, Cotton said. But, at least in the case of PKS 2014−55, that’s one cosmic treasure map solved.

Originally published on Live Science.

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  • Stephen J. Bauer
    So not only are some scientists providing analogies that black holes eat, but now some other scientists would like us to believe that some black holes even burp, or belch out matter. And then they identify the belched matter as 'X' shaped galaxy. I believe these nonsensical analogies are actually distorting the layperson's understanding of black holes. I'm sorry, but black holes and all large gravitationally dense objects rotate. This is something else.
    Reply
  • Nick Wilmshurst
    Beautiful image from MeerKAT. FYI this is an hour glass shape, not two boomerangs. It is a common formation, commonly misinterpreted . It is more frequently noticed in plantary nebula but exists in the galactic scale as well. It is a plasma phenomenon that can be reproduced down here on earth.
    Reply
  • sagitarriatjefferspin
    The article is imprecise and ambiguous, talking about the black holes "belching" matter into space.
    Is this matter coming from "inside" the black hole (which would be an astonishing finding that seemingly defies physics, going far beyond mere Hawking radiation), or from near the black hole/the accretion disc?
    Reply
  • Runesmith
    @stephen J Bauer: "So not only are some scientists providing analogies that black holes eat, but now some other scientists would like us to believe that some black holes even burp, or belch out matter."
    The words "belching" and "indigestion" are incredibly stupid ones to use in this situation, and does create confusion (or maybe the writer didn't understand the concept).

    Supermassive blackholes are surrounded by a swirling disk of dust and gas called an accretion disk. This disk gets hotter as it gets close to the black holes event horizon (the distance from the gravitational center where the escape velocity equals the speed of light). The temperature of the accretion disk close to the event horizon is many times hotter than the surface of stars.

    If a star strays close to the black hole, it can get shredded and suddenly a glut of material hits the accretion disk. When this material collides with the super heated disk, part of it erupts outwards at close to the speed of light, in narrow jets perpendicular to the plane of the accretion disk. These jets then collide with gas and dust, producing x-rays and radio waves.

    So, no matter comes out from the event horizon of the black hole. From our current understanding of physics, nothing can. The matter, in this case, is blasted out from the accretion disk.
    Reply
  • Runesmith
    Nick Wilmshurst said:
    Beautiful image from MeerKAT. FYI this is an hour glass shape, not two boomerangs. It is a common formation, commonly misinterpreted . It is more frequently noticed in plantary nebula but exists in the galactic scale as well. It is a plasma phenomenon that can be reproduced down here on earth.

    Please explain the plasma processes that can create an hourglass shape at galactic scales. Things you can demonstrate at centimeter scales don't do well when you extrapolate them to galactic scales. You probably know that electromagnetic field falls off as 1/r^3, right?
    Reply
  • Ogamol
    I'd call it 4 bursts, and assume a wobbling (precessing?) SMBH. The longest is the oldest; the "boomerang" is second oldest; the bright bulges just outside the galaxy is the 3rd oldest (second newest); and the white hourglass in the center is the newest. So, 4 stars eaten at 4 different times. (As the intergalactic medium is apparently sparse, it is unlikely to push back on anything. It may slow a burst over time, assuming merely cumulative inertial effects. But "pushback" would require massive magnetic field interactions or extremely long term gravitic pulling.)
    Reply
  • Sally
    Runesmith said:
    Please explain the plasma processes that can create an hourglass shape at galactic scales. Things you can demonstrate at centimeter scales don't do well when you extrapolate them to galactic scales. You probably know that electromagnetic field falls off as 1/r^3, right?
    Have a look here https://spaceweathernews.com/plasma/
    Reply
  • Runesmith
    Sally said:
    Have a look here https://spaceweathernews.com/plasma/
    Ok, I am probably stupid, but I can't find the answer on how plasma processes create an hourglass shape at galactic scales in that rambling mess of misinformation you linked to. Could you be more specific, please?
    Reply
  • Sally
    Now you've asked the question you are sure to find the answer, hopefully this is not beneath you hWNd7nogt3EView: https://www.youtube.com/watch?v=hWNd7nogt3E&list=PLwOAYhBuU3UfgHHzUZ-Ss4CY_YWIu39Nu&index=3
    Reply
  • Sally
    Runesmith said:
    Ok, I am probably stupid, but I can't find the answer on how plasma processes create an hourglass shape at galactic scales in that rambling mess of misinformation you linked to. Could you be more specific, please?
    I'm tending toward https://en.wikipedia.org/wiki/Hannes_Alfv%C3%A9n
    Reply