Space & Science

Strange radio sources in a distant galaxy cluster challenge our understanding

The Universe is littered with clusters of galaxies – huge structures stacked at the intersections of cosmic web. A single cluster can span millions of light-years and be made up of hundreds or even thousands of galaxies.

However, these galaxies represent only a few percent of the total mass of a cluster. About 80 percent of it is black matterand the rest is a “soup” of hot plasma: gas heated to over 10,000,000℃ and interwoven with weak magnetic fields.

We and our international team of colleagues have identified a series of rarely seen radio objects – a radio relic, a radio halo and a fossil radio emission – within a particularly dynamic cluster of galaxies called Abell 3266. They challenge existing theories on the origins of such objects and their characteristics.

Abell3266 4(Christopher Riseley, using data from ASKAP, ATCA, XMM-Newton and Dark Energy Survey)

Above: The colliding cluster Abell 3266 seen across the electromagnetic spectrum, using data from ASKAP and ATCA (red/orange/yellow colors), XMM-Newton (blue) and the dark energy Survey (background map).

Relics, aureoles 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 particles of the hot plasma, generating radio emissions. And this show comes in a variety of shapes and sizes.

One example is “radio relics”. They are arc-shaped and sit on the periphery of a cluster, powered by shock waves passing 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 aircraft breaks the sound barrier.

“Radio halos” are irregular sources located towards the center of the cluster. They are powered by turbulence in the hot plasma, which energizes the particles. We know that halos and relics are generated by collisions between clusters of galaxies – but many of their grainy details remain elusive.

Then there are the “fossil” radio sources. These are the radio remains 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 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.

Abel 3266

Our new paperpublished in the Royal Astronomical Society Monthly Noticespresents a very detailed study of a cluster of galaxies called Abell 3266.

It is a particularly dynamic and disorderly collision system about 800 million light-years away. It has all the characteristics of a system that should to be host to relics and halos – but none had been detected until recently.

Follow-up of the work carried out using the Murchison Widefield Array earlier This yearwe used new data from the ASKAP radio telescope and the Australian Telescope Compact Array (ATCA) to see Abell 3266 in more detail.

Our data paints a complex picture. You can see it in the main image: the yellow colors indicate the elements where the energy supply 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 lost a lot of energy over time or they never had much to start with.

The radio relic is visible in red near the bottom of the image (see below for zoom). And our data here reveals special characteristics that have never been seen before in a relic.

Abell3266 4(Christopher Riseley, using data from ASKAP, ATCA, XMM-Newton and Dark Energy Survey)

Above: Abell 3266’s “upside-down” relic is shown here with yellow/orange/red colors representing radio brightness.

Its concave shape is also unusual, earning it the catchy nickname “upside down” 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 remains of a supermassive black hole

The radiofossil, seen towards the upper right corner 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 lower left galaxy, with a central black hole that has long been extinct.

Abell3266 4(Christopher Riseley, using data from ASKAP, ATCA, XMM-Newton and Dark Energy Survey)

Above: The radio fossil of Abell 3266 is shown here with red colors and outlines representing the radio luminosity measured by ASKAP, and blue colors showing the hot plasma. The cyan arrow points to the galaxy that we believe once fed the fossil.

Our best physical models simply cannot match the data. This reveals gaps in our understanding of the evolution of these sources, gaps that we are working to fill.

Finally, using a clever algorithm, we defocused the main image to look for a very faint, invisible high-resolution emission, unearthing the first detection of a radio halo in the Abell 3266 (see below).

Abell3266 4(Christopher Riseley, using data from ASKAP, ATCA, XMM-Newton and Dark Energy Survey)

Above: Abell 3266’s radio halo is shown here with red colors and outlines representing the radio brightness measured by ASKAP, and blue colors showing the hot plasma. The dotted cyan curve marks the outer limits of the radio halo.

To the future

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

The telescopes we’ve used have been laying the groundwork for groundbreaking science since the square kilometer network project. Studies like ours allow astronomers to find out what we don’t know, but you can be sure we’ll find out.

We acknowledge the Gomeroi people as the traditional owners of the site where the ATCA is located, 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. The conversation

Christopher RiseleySearcher, University of Bologna and Tessa VernstromPrincipal Investigator, The University of Western Australia.

This article is republished from The conversation under Creative Commons license. Read it original article.

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