We found some strange radio sources in a distant galaxy cluster. They're making us rethink what we thought we knew.

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Galaxy clusters allow us to study a broad range of rich processes -
including magnetism and plasma physics - in environments we can't
recreate in our labs. The colliding cluster Abell 3266 as seen across
the electromagnetic spectrum, using data from ASKAP and the ATCA
(red/orange/yellow colours), XMM-Newton (blue) and the Dark Energy
Survey (background map). (Image credit: Christopher Riseley (Università
di Bologna),)

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

However, these galaxies represent only a few percent of a
cluster's total mass. About 80% of it is dark matter, and the rest
is a hot plasma "soup": gas heated to above 10,000,000 degrees Celsius
and interwoven with weak magnetic fields.

We and our international team of colleagues have identified a series 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 defy existing theories about both the origins of such
objects and their characteristics.

Relics, haloes and fossils

Galaxy clusters allow us to study a broad range of rich processes -
including magnetism and plasma physics - in environments we can't
recreate in our labs.

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

"Radio relics" are one example. They are arc-shaped and sit towards a
cluster's outskirts, powered by shockwaves travelling through the
plasma, which cause a jump in density or pressure, and energize the
particles. An example of a shockwave on Earth is the sonic boom that
happens when an aircraft breaks the sound barrier.

"Radio haloes" are irregular sources that lie towards the
cluster's center. They're powered by turbulence in the hot
plasma, which gives energy to the particles. We know both haloes and
relics are generated by collisions between galaxy clusters - yet many
of their gritty details remain elusive.

Then there are "fossil" radio sources. These are the radio leftovers
from the death of a supermassive black hole at the center of a radio
galaxy.

When they're in action, black holes shoot huge jets of plasma far
out beyond the galaxy itself. As they run out of fuel and shut off, the
jets begin to dissipate. The remnants are what we detect as radio
fossils.

Abell 3266

Our new paper, published in the Monthly Notices of the Royal
Astronomical Society, presents a highly detailed study of a galaxy
cluster called Abell 3266.

This is a particularly dynamic and messy colliding system around 800
million light-years away. It has all the hallmarks of a system
that should be host to relics and haloes - yet none had been detected
until recently.

Following up on work conducted using 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 Abell
3266 in more detail.

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

Redder colors show features that are only visible at lower frequencies.
This means these objects are older and have less energy. Either they
have 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 in Abell 3266 is shown here with
yellow/orange/red colours representing the radio brightness. (Image
credit: Christopher Riseley, using data from ASKAP, ATCA, XMM-Newton
and the Dark Energy Survey))

Its concave shape is also unusual, earning it the catchy moniker of a
"wrong-way" relic. Overall, our data break our understanding of how
relics are generated, and we're still working to decipher the complex
physics behind these radio objects.

Ancient remnants of a supermassive black hole

The radio fossil, seen towards 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 the
lower left, with a central black hole that has long been switched off.

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 powered the fossil (Image credit: Christopher Riseley, using
data from ASKAP, XMM-Newton and the Dark Energy Survey)

Our best physical models simply can't fit the data. This reveals gaps
in our understanding of how these sources evolve - gaps that we're
working to fill.

Finally, using a clever algorithm, we de-focused the lead image to look
for very faint emission that's invisible at high resolution,
unearthing the first detection of a radio halo in Abell 3266 (see
below).

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. (Image credit: Christopher Riseley, using data from
ASKAP, XMM-Newton and the Dark Energy Survey)

Toward the future

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

The telescopes we used are laying the foundations for revolutionary
science from the Square Kilometre Array project. Studies like ours
allow astronomers to figure out what we don't know - but you can be
sure we're going to find out.

We acknowledge 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.

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

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https://www.space.com/strange-radio-sources-from-galaxy-cluster/
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