The universe is full of galaxy clusters: huge structures piled up at the intersections 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 small percentage of the total mass of a cluster. About 80% is dark matter, and the rest is a hot plasma “soup” – gas heated above 10,000,000 ℃ and intertwined with weak magnetic fields.
We and our international team of colleagues have identified a number of rarely observed radio objects (a radio relic, a radio halo and fossil radio emission) within a particularly dynamic galaxy cluster called Abell 3266. They challenge existing theories about the two origins of these objects. and its characteristics.
Relics, Halos and Fossils
Galaxy clusters allow us to study a wide range of rich processes, including magnetism and plasma physics, in environments that we cannot recreate in our laboratories.
When the clusters collide with each other, large amounts of energy are injected into the hot plasma particles, generating radio emissions. And this emission comes in a variety of shapes and sizes.
“Radio Relics” is an example. They are arc-shaped and sit towards the outskirts of a cluster, powered by shock waves traveling through the plasma, which cause a jump in density or pressure and energize the particles. An example of a shock wave on Earth is the sonic boom that occurs when an airplane breaks the sound barrier.
“Radio haloes” are irregular sources found towards the center of the cluster. They are powered by the turbulence of the hot plasma, which gives energy to the particles. We know that both haloes and relics are generated by collisions between galaxy clusters, but many of their gritty details remain elusive.
Then there are “fossil” radio sources. These are the radio remnants of the death of a supermassive black hole at the center of a radio galaxy.
When in action, black holes shoot large jets of plasma far beyond the galaxy itself. As they run out of fuel and shut down, the jets begin to dissipate. The remains are what we detect as radiofossils.
Read more: Explainer: radio astronomy
Bee 3266
Our new paper, published in Monthly Notices of the Royal Astronomical Society, presents a very detailed study of a galaxy cluster called Abell 3266.
This is a particularly dynamic and messy collision system about 800 million light years away. It has all the hallmarks of a system that should host relics and halos, but none had been detected until recently.
Following work with the Murchison Widefield Array earlier this year, we used new data from the ASKAP radio telescope and the Australia Telescope Compact Array (ATCA) to look at Abell 3266 in more detail.
Our data paint a complex picture. You can see this in the main image: the yellow colors show the features where the energy input is active. The blue haze represents hot plasma, captured at X-ray wavelengths.
Redder colors show features that are only visible at lower frequencies. This means that these objects are older and have less energy. Either they’ve lost a lot of energy over time, or they never had much to begin with.
The radio relic is visible in red near the bottom of the image (see below for a zoom). And our data here reveal particular features that have never been seen before in a relic.
The “wrong way” relic of Abell 3266 is shown here in yellow/orange/red colors representing the radio brightness. Christopher Riseley, using data from ASKAP, ATCA, XMM-Newton and Dark Energy Survey)
Its concave shape is also unusual, earning it the catchy nickname of a “wrong way” relic. Overall, our data shatters our understanding of how relics are generated, and we are still working to decipher the complex physics behind these radio objects.
Ancient remnants of a supermassive black hole
The radiofossil, seen towards the upper right of the main image (and also below), is very faint and red, indicating that it is ancient. We believe that this radio emission originally came from the lower-left galaxy with a long-dead central black hole.
The radio fossil Abell 3266 is shown here with red colors and outlines representing the radio brightness measured by ASKAP, and blue colors showing the hot plasma. The cyan arrow points to the galaxy we think once fed the fossil. Christopher Riseley, using data from ASKAP, XMM-Newton and the Dark Energy Survey
Our best physical models simply do not fit the data. This reveals gaps in our understanding of how these sources evolve, gaps we are working to fill.
Finally, using a clever algorithm, we defocused the main image to look for very faint emission that is invisible at high resolution, unearthing the first detection of a radio halo in Abell 3266 (see below).
Abell 3266’s radio halo is shown here with red colors and contours representing the radio brightness measured by ASKAP, and blue colors showing the hot plasma. The dashed cyan curve marks the outer limits of the radio halo. Christopher Riseley, using data from ASKAP, XMM-Newton and the Dark Energy Survey
Towards the future
This is the beginning of the road to understanding Abell 3266. We have discovered a wealth of new and detailed information, but our study has raised even more questions.
The telescopes we used are laying 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 be sure we will find out.
We recognize the Gomeroi as the traditional owners of the site where ATCA is located, and the Wajarri Yamatji as the traditional owners of the Murchison Radio Astronomy Observatory site where ASKAP and the Murchison Widefield Array are located.