We found some strange radio sources in distant galaxy clusters. They make us rethink what we think we know.
The universe is filled with galaxy clusters – massive structures piled up at the intersection of the cosmic web. A single cluster can span millions of light years and consist of hundreds, or even thousands, of galaxies.
However, these galaxies represent only a few percent of the total mass of the cluster. About 80% of it is dark matter, and the rest is hot plasma “soup”: gas heated to above 10,000,000℃ and entwined with weak magnetic fields.
We and our team of international colleagues have identified a series of rarely observed radio objects – radio remains, radio halos, and radio emission fossils – in a highly dynamic galaxy cluster called Abell 3266. They challenge existing theories about the object’s origin. and their characteristics.
Relics, halos and fossils
Clusters of galaxies allow us to study a rich variety of processes – including magnetism and plasma physics – in environments that we cannot recreate in our laboratories.
When the clusters collide with each other, a large amount of energy is fed into the hot plasma particles, resulting in radio emission. And these emissions come in all shapes and sizes.
“Relic radio” is one such example. They are arc-shaped and sit towards the periphery of the cluster, supported by shock waves traveling through the plasma, which cause spikes in density or pressure, and energize the particles. An example of a shock wave on Earth is the sonic boom that occurs when a plane breaks through a sound barrier.
“Radio hello” is an irregular source located towards the center of the cluster. They are powered by turbulence in the hot plasma, which energizes the particles. We know both halos and relics are produced by collisions between galaxy clusters – but many of the gritty details remain elusive.
Then there is the “fossil” radio source. This is a radio remnant from the death of the supermassive black hole at the center of the radio galaxy.
As they act, the black holes shoot massive jets of plasma far beyond the galaxy itself. When they run out of fuel and die, the jets begin to disappear. It is these remains that we detect as radio fossils.
Read more: Explanation: radio astronomy
Abel 3266
Our new paper, published in the Monthly Notices of the Royal Astronomical Society, presents a very detailed study of the galaxy cluster called Abell 3266.
This is a very dynamic and messy collision system that is about 800 million light years away. It has all the advantages of a system that Should host relics and halos – but none have been detected to date.
Following up on work done using the Murchison Widefield Array earlier this year, we used new data from the ASKAP radio telescope and the Australian Telescope Compact Array (ATCA) to take a closer look at Abell 3266.
Our data paint a complex picture. You can see this in the main image: the yellow color indicates the feature where the energy input is active. The blue mist represents hot plasma, captured at X-ray wavelengths.
Redder colors indicate features that are only visible at lower frequencies. This means these objects are older and have less energy. Either they have lost a lot of energy over time, or they never had much energy to begin with.
The radio remains are visible in red near the bottom of the image (see below to enlarge). And our data here reveals certain features that have never been seen before in the relic.
Christopher Riseley, using data from ASKAP, ATCA, XMM-Newton and the Dark Energy Survey)
Its concave shape is also unusual, making it an attractive moniker of a “misguided” relic. Overall, our data break our understanding of how the relics were generated, and we are still working to decipher the complex physics behind these radio objects.
The ancient remains of a supermassive black hole
The radio fossil, seen at the top right of the main image (and also below), is very faint and red, indicating that it is ancient. We believe this radio emission originally came from the galaxy in the lower left, with the central black hole that has long been turned off.
Christopher Riseley, using data from ASKAP, XMM-Newton, and the Dark Energy Survey
Our best physical model cannot load data. This reveals a gap in our understanding of how these resources evolve – a gap we are working to fill.
Finally, using a clever algorithm, we defocused the main image to look for very faint emission that is not visible at high resolution, finding the first detection of a radio halo in Abell 3266 (see below).
Christopher Riseley, using data from ASKAP, XMM-Newton, and the Dark Energy Survey
To the future
This is the beginning of the path to understanding Abell 3266. We have discovered a lot of new and detailed information, but our research has raised many more questions.
The telescope we use laid the groundwork for the revolutionary science of the Square Kilometer Array project. Studies like ours allow astronomers to find out what we don’t know – but you can rest assured that we will.
We recognize the Gomeroi community as the traditional owners of the site where ATCA is located, and the Wajarri Yamatji community as the traditional owners of the Murchison Radioastronomy Observatory site, where ASKAP and the Murchison Widefield Array are located.
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