Why is this galaxy shaped like a boomerang?

X marks the spot: New observations reveal the hidden treasure of strange “X-galaxies.”
By | Published: May 13, 2020 | Last updated on May 18, 2023
The strange, boomerang-shaped galaxy PKS 2014-55 has radio jets (blue) spewing from its black hole that appear curved, rather than straight.
Like a pirate attempting to find treasure, astronomers from South Africa and the United States followed the “X” to reach the answer to a mysterious phenomenon happening in deep space.

In galaxies with an active supermassive black hole, astronomers often see twin jets erupting from their center. These jets typically spew outward into space in opposite directions. But in the galaxy PKS 2014-55, about 800 million light-years from Earth, the jets coming from its central supermassive black hole don’t act like this. Instead, this galaxy — and other “X-galaxies” like it — appears to have four jets forming the shape of an “X.”

The reason why X-galaxies have such strange shapes was previously unknown. But thanks to new detailed MeerKAT observations, astronomers have found the explanation. PKS 2014-55’s X-shaped jets of radio waves, which extend 2.5 million light-years into space, are being turned back onto the galaxy as they encounter the pressure of intergalactic gas. But as the material falls back toward the center of the galaxy, it’s deflected by higher gas pressure near the center and instead curves outward, creating the horizontal arms of the X.

As gas from the galaxy’s radio jets shoots outward, it eventually slows and stops, then falls back toward the center. On the way, it’s deflected, creating the curved, X-shaped jets astronomers observe.
Before these findings came out, astronomers had several theories about the processes behind the shape of X-galaxies. Some thought the jets changed direction over time as the spin of the black hole changed. Others suspected perhaps two supermassive black holes in the center, indistinguishable to telescopes, were spewing out two different pairs of jets in different directions. Now, though, the mystery has been solved, revealing an interesting battle between the black hole’s jets and the gas through which they’re moving.

The paper has been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society.