The universe is dotted with galactic clusters—huge structures clustered at the intersections of space network. 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 percent of it is dark matterand the rest is a hot plasma “soup”: gas heated to over 10,000,000℃ and laced with weak magnetic fields.
We and our international team of colleagues have identified a series of rarely observed radio objects – a radio relic, radio halo and fossil radio emission – within a particularly dynamic galaxy cluster called Abell 3266. They challenge existing theories about both the origin of such objects and their characteristics.
Top: The colliding cluster Abell 3266 as seen in the electromagnetic spectrum using data from ASKAP and ATCA (red/orange/yellow), XMM-Newton (blue) and Dark energy Survey (background map).
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, huge amounts of energy are put into the hot plasma particles, generating radio emission. And this feed comes in different shapes and sizes.
“Radio Relics” is one example. They are arc-shaped and located towards the outskirts of the cluster, powered by shock waves passing through the plasma, which cause a jump in density or pressure and charge the particles with energy. 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 that lie towards the center of the cluster. They are powered by turbulence in the hot plasma, which energizes the particles. We know that both haloes and relics are generated by collisions between galaxy clusters – but many of their coarse 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 they are in action black holes shoot huge jets of the plasma far beyond the galaxy itself. When they run out of fuel and shut down, the nozzles start to dissipate. The remains are what we find as radio fossils.
Ours new paperpublished in Monthly Notices of the Royal Astronomical Societypresents an extremely detailed study of a galaxy cluster called Abell 3266.
It is a particularly dynamic and chaotic colliding system about 800 million light-years away. It has all the hallmarks of a system that Must to host relics and halos – but none were discovered until recently.
Our data paint a complex picture. You can see this in the lead image: the yellow colors indicate functions where the energy input is active. The blue haze represents the hot plasma captured at X-ray wavelengths.
Redder colors indicate features seen only 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 at the bottom of the image (see below for magnification). And our data here reveal specific features never before seen in a relic.
Above: The ‘wrong-way’ relic in Abell 3266 is shown here with yellow/orange/red colors representing the radio brightness.
Its concave shape is also unusual, earning it the catchy nickname of a “wrong” relic. Overall, our data disrupts our understanding of how the 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 in the upper right corner of the lead image (and also below) is very pale and red, indicating that it is ancient. We believe that this radio emission originally comes from the galaxy in the lower left, with a central black hole that has long since been turned off.
Above: The radio fossil in Abell 3266 is shown here with red colors and contours depicting 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.
Our best physical models simply cannot accommodate the data. This reveals gaps in our understanding of how these sources develop—gaps that we are working to fill.
Finally, using a clever algorithm, we defocused the lead image to look for very faint emission that is invisible at high resolution, making the first detection of a radio halo in Abell 3266 (see below).
Above: The radio halo in Abell 3266 is shown here with red colors and contours depicting 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.
To the future
This is the beginning of the road to understanding Abell 3266. We discovered a wealth of new and detailed information, but our research raised even more questions.
The telescopes we used laid the groundwork for the revolutionary science of A square kilometer array project. Research like ours allows astronomers to understand what we don’t know—but you can be sure we will.
We recognize the Gomeroi people as the traditional owners of the ATCA site and the Wajarri Yamatji people as the traditional owners of the Murchison Radio Astronomy Observatory site where ASKAP and the Murchison Widefield Array are located.