A team of astronomers from South Africa and the USA has used the MeerKAT telescope to solve a longstanding puzzle in ‘X’-shaped radio galaxies by observing a galaxy called PKS 2014-55 that’s situated 800 million light years away from Earth. Previous studies of these unusual galaxies lacked the high-quality imaging provided by the MeerKAT telescope, which was inaugurated in 2018.

This study was carried out by a team from the South African Radio Astronomy Observatory (SARAO), the (US) National Radio Astronomy Observatory (NRAO), the University of Pretoria, and Rhodes University. The results will be published in the journal Monthly Notices of the Royal Astronomical Society.

Dr Kshitij Thorat, a postdoctoral fellow at the University of Pretoria and second author of the paper, explained: "This galaxy is one of many with similar X-shaped morphologies carefully chosen to be studied in a MeerKAT observation campaign which is aimed at solving the mystery of X-shaped radio galaxies. While other telescopes around the world hinted at this object's unusual morphology, it took the crystal-clear image quality of MeerKAT to reveal the underlying physical causes. As a bonus, we have produced one of the most beautiful radio images I have ever seen." 

The image shows two powerful jets of radio waves, indicated in blue colour, each extending 2.5 million light-years (comparable to the distance between the Milky Way and the Andromeda galaxy, our nearest major neighbour). At their centre are the youngest jets of a central black hole, surrounded by an oblong disk of stars. The detail provided in this radio image obtained with the MeerKAT telescope shows that its shape is best described as a “double boomerang”.

Many galaxies far more active than the Milky Way have enormous twin jets of radio waves extending far into intergalactic space. Normally these go in opposite directions, coming from a massive black hole at the centre of the galaxy. However, a few are more complicated and appear to have four jets forming a mysterious ‘X’ on the sky.

Several possible explanations have been proposed to understand this phenomenon. These include changes in the direction of spin of the black hole at the centre of the galaxy and its associated jets; or two separate black holes, each associated with a pair of jets; or a third explanation of material falling back into the galaxy being deflected into different directions forming the other two arms of the ‘X’.

The new MeerKAT observations of PKS 2014-55 strongly favour the third explanation, as they show material “turning the corner” as it flows back towards the host galaxy. The arms or wings of the X are “turned back” by the pressure of low-density intergalactic gas. As they flow back towards the central galaxy, they are deflected by its relatively high gas pressure into the shorter, horizontal, arms of the boomerang.

Professor Roger Deane, who leads the UP astronomy group and is a co-author of the study, said, “Here at the University of Pretoria, we've made a concerted effort over the past two years to build a team that is able to make important scientific discoveries with cutting-edge radio telescopes. The rapid growth we've seen is both gratifying and encouraging, particularly with the exquisite images the team is making with South Africa's MeerKAT telescope, a precursor to the Square Kilometre Array.” This study is part of an ongoing effort at the University of Pretoria to carry out systematic studies of X-shaped galaxies using next-generation telescopes like MeerKAT and techniques like machine learning, which would find similar but more faraway objects in the universe.

The MeerKAT telescope array consists of 64 radio dishes located in the Karoo semi-desert in the Northern Cape province of South Africa. To make this image, computers combined over a petabyte of data (equivalent to over 100 000 MP3 audio files) from these antennas into a telescope 8 km in diameter.

Lead author William Cotton of the NRAO added, “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.” Further research into these open questions is already underway.

Bernie Fanaroff, co-author of the study and former director of the SKA South Africa project that built MeerKAT, noted that “MeerKAT was designed to be the best of its kind in the world. It’s wonderful to see how its unique capabilities are contributing to resolving long-standing questions related to the evolution of galaxies.”

The authors of the study are: William Cotton, Kshitij Thorat, Jim Condon, Bradley Frank, Gyula Józsa, Sarah White, Roger Deane, Nadeem Oozeer, Marcellin Atemkeng, Landman Bester, Bernie Fanaroff, Sydil Kupa, Oleg Smirnov, Tom Mauch, Vasaant Krishnan and Fernando Camilo.

Annotated image that illustrates how the X-shape is formed and how a new, young pair of jets have just started up near the supermassive black hole at the heart of the galaxy (inset). Image credit: UP, original image: NRAO/AUI/NSF; SARAO; DES

DOWNLOAD Image of PKS 2014-55  Image Credit: NRAO/AUI/NSF; SARAO;DES