ASKAP, MeerKAT and EMU are revealing a new low surface brightness universe of radio circles

Radio astronomers can detect what our eyes never will. By exploring the heavens with telescopes that capture radio waves instead of visible light, we often find ourselves looking at circles.

A new wave of radio facilities - notably the Australian Square Kilometre Array Pathfinder (ASKAP) and MeerKAT in South Africa - is now bringing extraordinarily faint cosmic targets into view for the first time.

Astronomers use surface brightness to describe how readily an object can be seen. Thanks to the remarkable sensitivity of MeerKAT and ASKAP, radio astronomers are now opening up a new "low surface brightness universe".

This realm is made up of radio sources so dim they have never previously been detected, and each one comes with its own distinctive physical characteristics.

Many of the ASKAP findings discussed here come from one of its flagship observing programmes: EMU (Evolutionary Map of the Universe).

EMU is surveying the whole southern sky with unprecedented sensitivity, and will produce the most detailed map of the Southern Hemisphere sky yet - a striking new radio atlas expected to underpin research for decades.

It is EMU’s near all-sky coverage, together with ASKAP’s outstanding sensitivity (particularly across the Milky Way), that has driven so many of these recent discoveries.

Here is what they are showing us.

Unstable stars

The faint ring Kýklos (from the Greek κύκλος, meaning circle or ring) and the object WR16 both reveal the surroundings of rare and unusual stars known as Wolf–Rayet stars.

As massive stars approach the end of their fuel supply, they become unstable and enter one of the final phases of stellar evolution, turning into Wolf–Rayet stars. They start to surge and pulsate, throwing off their outer layers - material that can create bright, cloudy structures around the star.

In these cases, an earlier episode of outflow has already cleared out the region around the star, enabling the current eruption to expand evenly in every direction. The resulting shell of stellar debris appears to us as a circle.

Exploded stars

Stingray 1, Perun, Ancora and Unicycle are supernova remnants. When a massive star ultimately exhausts its fuel, it can no longer resist gravity’s inward crush. Collapsing matter triggers one last explosion, and the remnants of these violent stellar deaths are known as supernovas.

As their shockwaves spread, they sweep surrounding material into an expanding sphere, producing striking circular patterns.

Over time, a remnant’s surroundings will distort it. If one side of the blast ploughs into an interstellar cloud, the outline can look flattened. That is why an almost perfect circle in a chaotic universe is such an exceptional find.

Teleios - named from the Greek Τελεɩοσ ("perfect") because its shape is so close to perfectly circular - is shown below. This unusual object has not been detected at any wavelength, including visible light, underscoring ASKAP’s ability to uncover entirely new phenomena.

Its form suggests Teleios has been comparatively unaffected by its environment. That gives us a chance to draw inferences about the original supernova blast, offering rare insight into one of the most energetic processes in the universe.

At the other end of the spectrum, we can revisit an object and learn something wholly new about it. The Diprotodon supernova remnant is shown below.

This remnant is among the largest objects in the sky, appearing roughly six times larger than the Moon. The name reflects that scale: Diprotodon was one of Australia’s best-known megafauna - a giant wombat that lived about 25,000 years ago.

ASKAP’s sensitivity has revealed the remnant’s full size. That in turn prompted more detailed analysis, bringing out further history and the physics driving this object. Its chaotic interior can be understood as sections of the expanding shell colliding with a crowded interstellar environment.

A cosmic mirror

Lagotis is another example of how new telescope observations can lead to a fresh classification for objects discovered earlier.

The reflection nebula VdB-80 has long been known within the plane of our Milky Way galaxy. The light we observe comes from nearby stars and is then reflected by a neighbouring cloud of gas and dust.

But using newly available ASKAP EMU observations, we were able to identify an associated cloud of ionised hydrogen (an HII region, pronounced "aitch two"), where energy from stars has stripped electrons from the gas.

This HII region appears alongside the reflection nebula, sharing the same stellar centre, and arises as the star presses into a molecular cloud. Because this motion resembles burrowing, the object was named Lagotis after Macrotis lagotis, the Australian greater bilby.

Outside the galaxy

ASKAP and MeerKAT are also bringing extragalactic objects into sharper focus - including so-called "radio ring" galaxies. In visible light, the stellar distribution in such a galaxy looks like a fairly ordinary disc.

In radio emission, however, a ring stands out. Why does the centre look hollow? One possibility is that the combined impact of many supernova explosions has pushed radio-emitting clouds out from the middle. We do not yet know - so we are seeking more examples to test these ideas.

Finally, LMC-ORC is an Odd Radio Circle (ORC), part of a striking new class of objects with unknown origins. Visible only at radio wavelengths, they may be the most enigmatic of all.

The next generation

MeerKAT and ASKAP are delivering remarkable new perspectives on the low surface brightness universe.

Even so, they are stepping stones towards the Square Kilometre Array - an international collaborative effort that will expand what radio astronomers can do, and expose even more unusual features of the cosmos.

The low-surface brightness universe is full of unanswered questions, and these discoveries are pushing our understanding onward. At present, the EMU survey with ASKAP is only 25% complete.

As additional survey data are released, we will find many more distinctive and exciting targets - both objects entirely new to astrophysics and new extensions of phenomena we already know.

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Acknowledgements: Aaron Bradley and Zachary Smeaton, Master’s research students at Western Sydney University, made valuable contributions to this article.

Miroslav Filipovic, Professor, Western Sydney University; Andrew Hopkins, Professor of Astronomy, Macquarie University; Luke Barnes, Senior Lecturer in Physics, Western Sydney University, and Nicholas Tothill, Associate Professor, Western Sydney University

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