Astronomers studying images of the Abell 7 galaxy cluster obtained by LOFAR, Europe’s largest radio interferometer, have discovered a very unusual X-shaped radio galaxy. Judging by the collapsed and asymmetric structures, the galaxy is dying.

Radio galaxies are among the brightest objects in their range in the universe. At their centers are supermassive black holes that absorb stars and dust. Much of the energy released goes into jets, giant jets of high-energy particles accelerated to relativistic speeds. Synchrotron radiation also occurs.

When the black hole is active, large structures such as the core, jets, hot spots and lobes can be distinguished in the radio galaxy. This stage lasts about 100 million years. Then the energy source is turned off and the structures gradually disappear. The galaxy is dying.

Finding such an object in the disappearing phase is a great achievement. Scientists from India and the UK were lucky: they discovered a radio galaxy with collapsed lobes in the Abell 7 galaxy cluster. An article about it was published on the Cornell University preprint website.

Radio lobes are large regions of plasma ejected by jets from the galactic core. They are arranged at angles to each other, forming an X shape.

Under normal conditions, two symmetric wings can be seen in X-shaped radio galaxies, but sometimes secondary wing structures also appear. This is a very rare and poorly understood phenomenon.

There are several hypotheses explaining the birth of secondary lobes. Perhaps it is due to the presence of a binary black hole system at the center, or the jets are changing direction. The most convincing explanation is that the lobes are “inflated” by a backflow of heated plasma. As the authors of the new study note, the radio galaxy J0011+3217 has only one secondary lobe, which makes it even more interesting.

Scientists made the discovery by studying the Abell 7 galaxy cluster using archival data from various ground-based and orbiting telescopes. Among the LOFAR survey images, an unusual X-shaped radio source was found – LoTSS DR2. LOFAR (International Low Frequency Array) is the largest ground-based radio interferometer operating at low frequencies in the 120-168 megahertz range. It was developed in the Netherlands and its instruments are located throughout Europe.

The radio galaxy J0011+3217 has two main lobes. A single secondary lobe extends 2.77 million light-years (0.85 megaparsecs) from its centre. Two hot spots are visible in the active nucleus region. The radio galaxy is also visible in the optical range, as observed by the Pan-STARRS1 telescope.

A few radio galaxies are known to have only a single secondary blade. In all, it is short, about one-fourth the size of the primary blade. In J0011+3217, the secondary giant dipper takes up 85 percent of the volume of the primary dipper. The blades are also slightly bent. This is known for jets in broad-tailed radio sources, which are often found in dense cluster regions. The bending structures are believed to be the result of the high-speed incursion of the Abell 7 cluster into this galaxy.

Scientists now plan to study the unusual radio galaxy in more detail in the optical and X-ray bands and with the help of simulations.