Strange radio sources in distant galaxy clusters are challenging our understanding

The universe is full of galactic clusters – huge structures stacked 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 cluster’s total mass. About 80 percent of it is dark matter, and the rest is hot plasma “soup”: a gas heated to more than 10,000,000 and entangled with weak magnetic fields.

We and our international team of colleagues have identified a series of rarely observed radio objects – radio remnants, radio corona, and fossil radio emissions – within a particularly dynamic group of galaxies called Abell 3266. They are challenging existing theories about the origin and properties of each of these objects.

(Christopher Risley, using data from ASKAP, ATCA, XMM-Newton, and the Dark Energy Survey)

Above: Collider group Abell 3266 as seen across the electromagnetic spectrum, using data from ASKAP and ATCA (red/orange/yellow colours), XMM-Newton (blue), and dark energy surveys (background map).

Monuments, 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, massive amounts of energy are put into the hot plasma particles, resulting in a radio emission. This emission comes in a variety of shapes and sizes.

One example is “radio traces”. They are arc-shaped and sit toward the outskirts of the mass, and are powered by shock waves traveling through the plasma, which cause a jump in density or pressure, activating the particles. One example of shock waves on Earth is the sonic boom that occurs when an aircraft breaks the sound barrier.

“Radio halos” are irregular sources located toward the center of the cluster. It is powered by turbulence in hot plasma, which gives energy to the particles. We know that both halos and effects are caused by collisions between galaxy clusters – yet many of their subtleties remain elusive.

Then there are “fossil” radio sources. These are radio remnants from the death of a supermassive black hole at the center of a radio galaxy.

When in working order, black holes shoot huge jets of plasma away from the galaxy itself. As the fuel runs out and shuts down, the jets start to waste away. The remains are what we discover as radio fossils.

Ebel 3266

Our new paper published in Monthly Notices of the Royal Astronomical Societypresents a very detailed study of a group of galaxies called Abell 3266.

This is a particularly dynamic and chaotic collision system about 800 million light-years away. It has all the distinguishing features of a system should It hosts relics and auras – yet none of them were discovered until recently.

Following up on work done 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 see the Abell 3266 in more detail.

Our data paints a complex picture. You can see this in the main image: the yellow colors show features where the power input is active. The blue haze represents hot plasma captured at X-ray wavelengths.

Red colors show features that only appear at lower frequencies. This means that these things are older and have less energy. Either they lost a lot of energy over time, or they didn’t have much to begin with.

Radio remnants are visible in red near the bottom of the image (see below to enlarge). Our data here reveals certain features not previously seen in relics.

Abel 3266 4(Christopher Risley, using data from ASKAP, ATCA, XMM-Newton, and the Dark Energy Survey)

Above: Abell 3266’s “wrong path” remnants are shown here in yellow/orange/red colors representing radio brightness.

Its concave shape is also unusual, making it an attractive nickname for the “wrong way” relic. Overall, our data shatters our understanding of how the effects are created, and we’re still working on deciphering the complex physics behind these radio objects.

Ancient remnants of a supermassive black hole

The radioactive fossil, seen in the upper right of the main image (and also below), is very faint and red, indicating that it is old. We believe this radio transmission originally came from the galaxy at the bottom left, with a central black hole long closed.

Abel 3266 4(Christopher Risley, using data from ASKAP, ATCA, XMM-Newton, and the Dark Energy Survey)

Above: The radio-fossil of Abell 3266 is shown here with colors and red lines depicting the radio brightness measured by ASKAP, and blue colors showing hot plasma. The celestial arrow points to the galaxy that we think once powered the fossil.

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 an intelligent algorithm, we defocused the main image to search for a very faint, invisible emission at high resolution, revealing the first detection of a radio corona in the Abell 3266 (see below).

Abel 3266 4(Christopher Risley, using data from ASKAP, ATCA, XMM-Newton, and the Dark Energy Survey)

Above: Abell 3266’s radio aura is shown here with colors and red lines depicting radio brightness measured by ASKAP, and blue colors showing hot plasma. The cyan dashed curve indicates the outer limits of the radio aura.

towards the future

This is the beginning of the path toward understanding Abell 3266. We’ve discovered a wealth of new and detailed information, but our study raises even more questions.

The telescopes we used lay the foundations for the revolutionary science of the Square Kilometer Array project. Studies like ours allow astronomers to discover what we don’t know – but you can be sure we will.

We acknowledge the Gomeroi people as the traditional owners of the site where the ATCA is located, and the Wajarri Yamatji people are the traditional owners of the Murchison Radioastronomy Observatory site, where the ASKAP and the Murchison Widefield Array are located. Conversation

Christopher Riselli, Research Fellow, University of Bologna and Tessa Fernstrom, Senior Research Fellow, University of Western Australia.

This article has been republished from The Conversation under a Creative Commons license. Read the original article.

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