Strange radio sources in distant galaxy clusters defy our understanding

The universe is dotted with galaxy clusters – huge structures that loom 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 make up only a few percent of the total mass of a cluster. About 80 percent of this is dark matter, the rest a hot plasma “soup”: gas heated to over 10,000,000 °C and laced 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 emissions – within a particularly dynamic galaxy cluster called Abell 3266. They refute existing theories about the origin of objects and their properties.

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

Top: 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, 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 replicate in our laboratories.

When clusters collide with each other, huge amounts of energy are injected into the hot plasma particles, producing radio emissions. And this emission comes in a variety of shapes and sizes.

“Radio Relics” is an example. They are arc-shaped and sit at the edge of a cluster, driven by shock waves traveling through the plasma, causing a density or pressure jump and energizing the particles. An example of a shock wave on Earth is the sonic boom that occurs when an airplane breaks the sound barrier.

“Radio halos” are irregular sources that lie toward the center of the cluster. They are powered by turbulence in the hot plasma, which gives the particles energy. We know that both halos and relics are produced by collisions between galaxy clusters – yet many of their gritty details remain elusive.

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

When in action, black holes shoot out huge jets of plasma far beyond the galaxy. When they run out of fuel and shut down, the jets begin to disintegrate. The remains are what we discover as radiofossils.

Abell 3266

Our new paper, published in Monthly Bulletins of the Royal Astronomical Societypresents a very detailed study of a galaxy cluster called Abell 3266.

This is a particularly dynamic and messy colliding system some 800 million light-years away. It has all the hallmarks of a system that should harbor relics and haloes – but none have been discovered until recently.

Following work done earlier this year with the Murchison Widefield Array, we used new data from the ASKAP radio telescope and the Australia Telescope Compact Array (ATCA) to get a closer look at Abell 3266.

Our data paint a complex picture. You can see this in the mission statement: Yellow colors indicate features where energizing is active. The blue veil represents the hot plasma captured at X-ray wavelengths.

Redder colors indicate features only visible at lower frequencies. This means that these objects are older and have less energy. They either lost a lot of energy over time, or they never had much to begin with.

The radio relic can be seen in red at the bottom of the image (see below for a zoom). And our data here shows special features never before seen in a relic.

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

Above: The “fake” relic in Abell 3266 is shown here with yellow/orange/red colors representing radio brightness.

Also unusual is its concave shape, which earned it the catchy nickname of a “wrong way”. Overall, our data is disrupting our understanding of how relics are created, and we’re still working to unravel the complex physics behind these radio objects.

Ancient remnants of a supermassive black hole

The radiofossil seen in the upper right of the lead image (and also below) is very faint and red, indicating it is ancient. We believe this radio emission originally came from the galaxy at lower left, with a central black hole that has long been switched off.

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

Above: The radiofossil in 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 believe once powered the fossil.

Our best physical models just don’t fit the data. This reveals gaps in our understanding of how these sources are evolving – gaps that we are working to fill.

Finally, using a clever algorithm, we defocused the lead image to look for a very faint emission that is invisible at high resolution, resulting in the first detection of a radio station in Abell 3266 (see below).

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

Above: The radio halo in Abell 3266 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.

towards the future

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

The telescopes we use lay the foundation for the revolutionary science of the Square Kilometer Array project. Studies like ours allow astronomers to find out what we don’t know – but rest assured that we will find out.

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

Christopher Riseley, Research Fellow, Università di Bologna and Tessa Vernstrom, Senior Research Fellow, The University of Western Australia.

This article was republished by The Conversation under a Creative Commons license. Read the original article.