Scientists crack the mystery of X-shaped galaxy

x shape galaxy
The MeerKAT image of the giant X-shaped radio galaxy PKS 2014-55. Credit: NRAO / AUI / NSF / SARAO / DES

A team of scientists from the USA and South Africa have published highly detailed images of the largest X-shaped “radio galaxy” ever discovered - PKS 2014-55.

The images have helped to solve long standing confusions about the galaxy’s unusual shape. 

Nearly all galaxies have an enormous black hole at their centre.

In an active galaxy, powerful jets of charged particles can emerge from the area around the black hole. Astronomers believe these are emitted from near the poles of the black hole, which is why there are two of them, and they usually point in opposite directions.

Typically radio galaxies only have one set of lobes - made up of a “jet” and “counter-jet”. 

These jets expand into the surrounding space at nearly the speed of light. They initially move in a straight line, but twist and bend into various different shapes as they move through their surroundings.

The jets that typically move in opposite directions, derive from an enormous black hole at the centre of a galaxy. However, astronomers have also concluded that some galaxies have two sets of lobes that form an ‘X’ like shape in the cosmos.

The distinct X-shape of the radio galaxy PKS 2014-55 is made up of two pairs of giant lobes consisting of hot jets of electrons.

Mystery solved...?

Previously there were two main theories surrounding why X-shaped phenomena formed in certain radio galaxies.

A black hole at the centre of the galaxy changing its spin direction and associated jets - over millions of years.
The presence of two black holes and therefore meaning two sets of lobes.

The recent observations from the South African MeerKAT telescope strongly suggest a third reason for the X-shape formation.  

The images of PKS 2014-55, an X-shaped radio galaxy located 800 million light-years away from Earth in the constellation of Telescopium, suggest that the two larger lobes present are fast-moving particles zooming out from the black hole, while the two smaller lobes are the backflow looping around to fall back in.

These jets extend over 2.5 million light years in space, a distance comparable to that of the Milky Way and the Andromeda galaxy.The high resolution images captured by MeerKAT are ten times more sensitive than the  Australian Square Kilometre Array Pathfinder (ASKAP) telescope pilot attempt in Australia, last year.

"MeerKAT was designed to be the best of its kind in the world," said Bernie Fanaroff, an astronomer at the South African Radio Astronomy Observatory.

"It's wonderful to see how its unique capabilities are helping to solve long standing questions about the evolution of galaxies.”

The sets of lobes emit electromagnetic radiation in the form of radio waves, which can only be detected by specialised radio telescopes. Although it is impossible to see the radio waves with the human eye, if it were possible - this radio galaxy would look about the same size as our Moon.

"MeerKAT is one of a new generation of instruments whose power solves old puzzles even when it finds new ones. 

PKS 2014-55 shows features never seen before in this detail which are not fully understood,” said William Cotton, an astronomer at the National Radio Astronomy Observatory and the South African Radio Astronomy Observatory.

Reference:

Cotton, W.D., et al., ‘Hydrodynamical Backflow in X-shaped Radio Galaxy PKS 2014-55’, Monthly Notices of the Royal Astronomical Society, staa1240, (7 May, 2020), https://doi.org/10.1093/mnras/staa1240

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