Radio Astronomers May Have Just Solved The Mystery Of X-Galaxies

The galaxy PKS 2014-55 as seen by MeerKAT. Credit: NRAO/AUI/NSF; SARAO; DES.

At the center of almost every galaxy there is a supermassive black hole and if a lot of stuff gets too close, the black hole can become active. The ensuing dramatic feeding process has many effects on the galaxy; among them is the emission of jets of particles stretching for many hundreds of thousands of light-years.

These jets tend to expand from the center of the galaxy outwards in opposite directions. But researchers have discovered that certain galaxies don’t follow this neat picture. They don’t have two jets but four, forming a galactic-sized "X" in the cosmos. An upcoming study due to be published in the Monthly Notices of the Royal Astronomical Society finally explains why.

Astronomers have three ideas to explain how such an X shape might form. One possibility is that over millions of years, the black hole shifts its position creating the double boomerang shape. Another considers two supermassive black holes, both active, creating the different branches of the X. The third has material falling back into the galaxy and then being deflected by high-pressure gas into different directions. According to the authors of the study, this third option appears to be the correct one.

The team of astronomers from the South African Radio Astronomy Observatory (SARAO), the NRAO, the University of Pretoria, and Rhodes University used South Africa's MeerKAT radio observatory to study X-shaped giant radio galaxy PKS 2014-55. Thanks to the incredible resolution of MeerKAT's 64 radio dishes, the team was able to confirm the direction of the jet very close to the black hole. The jet is aligned with the long arm of the X and the team was also able to track the material that had been previously shot out by the jet over million of years. Some of that material falling back into the galaxy and encountering the hotter, newer part of the jet is pushed aside in a different direction. This material "turning the corner" creates the other two arms of the X.

Annotated image showing X-shaped giant radio galaxy PKS 2014-55, indicating the old X-shaped radio jets, the younger jets closer to the central black hole, and the region of influence dominated by the central galaxy’s stars and gas. The curved arrows denote the direction of the backflow that forms the horizontal components of the X. Credit: UP; NRAO/AUI/NSF; SARAO; DES.

“MeerKAT is one of a new generation of instruments whose power solves old puzzles even as it finds new ones – this galaxy shows features never seen before in this detail, which are not fully understood,” lead author William Cotton of the National Radio Astronomy Observatory (NRAO) said in a statement. MeerKAT is located in the Karoo semi-desert in South Africa. 

Galaxy PKS 2014-55 is located 800 million light-years from Earth and its peculiar jets extend for 2.5 million light-years into space, a distance comparable to the separation between the Milky Way and the Andromeda galaxy. 

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