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6 mins read 19 Mar 2020

The most colossal bang since the biggest one!

Astronomers observe one of the largest explosions in the history of the Universe - using a number of different telescopes, including Australia’s MWA observatory.

Composite image of the hot gas inside the Ophiuchus cluster. Credit: x-ray: Chandra; NASA/cxc/nrl/S. Giacintucci, et al., XMM: ESA/XMM., Radio: NCRA/TIFR/GMRT., Infrared: 2MASS/UMASS/Caltech/NASA/NSF

Our universe is a violent place, with immense exploding stars tearing themselves apart, and merging black holes sending ripples of space and time across the cosmos. Recently astronomers have observed an immense scar on the sky, the tell-tale evidence for one of the largest explosions to have occurred over the history of the universe.

The astronomers were exploring an unexpected hole in the Ophiuchus cluster of galaxies, located about 390 million light-years from Earth. Such clusters can be home to many hundreds of individual galaxies, bound together by the gravitational influence of an enveloping distribution of dark matter. Between the galaxies sits hot gas, held in place and heated by the gravitational squeezing of this unseen mass. This squeezing is so intense that the hot gas begins to glow in x-rays, a sure sign of the presence of dark matter.

In Ophiuchus, the peak of this x-ray emission is near the most massive cluster galaxy, which is expected as it is here as the deepest part of the dark matter’s gravitational potential, with surrounding emission extending more than one and a half million light-years throughout the cluster. Except astronomers noticed that on one side of the cluster, there appeared to be an enormous hole in the x-rays, an immense one and a half million light-year cavity that lacked the hot gas seen in the rest of the cluster.

Radio contours overlaid on the x-ray image that shows the cluster structure on larger scales. The cluster shows elongation on the side of the radio lobe. The green arc traces the x-ray edge. Credit: MWA GLEAM/ROSAT/Chandra/XMM.

Recently, astronomers turned their radio telescopes towards the Ophiuchus cluster, including the Murchison Widefield Array (MWA) in Western Australia, Giant Metre-wave Radio Telescope (GMRT) in India, and Very Large Array (VLA) in the USA. Unlike the hot, dense gas seen in x-rays, these radio observations would reveal more presence of more rarefied, less energetic gas, dumped there by an earlier event and cooling ever since. 

Science Check: What are Galaxy Clusters and Dark Matter?

When we look out into the night sky with our eyes, all the stars that we can see - reside in our own Galaxy, the Milky Way. Using instruments like binoculars and telescopes, we can see a lot more of the stars in our Galaxy - and even out beyond our Milky Way, to other galaxies. 

Whilst galaxies are their own structures, they do reside in larger organisations known as Galaxy Clusters. These clusters usually have hundreds to thousands of galaxies that are mutually bound by gravitation. 

The Milky Way itself forms part of a more local community of galaxies, known as the Local Group - and looking at a grander scale, the Local Group forms part of a bigger collective known as the Virgo Supercluster.

Diagram of the Local Group of galaxies, relative to the Milky Way. Credit: Andrew Z. Colvin.

The composition of Galaxy Clusters is usually made up of a small percentage of the galaxies we see with our telescopes, roughly around 1% (this is still huge, given each galaxy often contains millions or billions of stars!). 

About 9% of Galaxy Clusters are made up of inter-galactic hot gas - the plasma that is kept together between galaxies and emits at x-rays. And the remaining 90% is caused by Dark Matter, which provides the gravitational force that binds the cluster together, accelerates the galaxies, and squeezes the hot gas to extreme temperatures. 

But what is dark matter?

Everything we see around us - stars, planets, gas, humans, hamburgers, buildings, etc. are made of ordinary matter (i.e. atoms), technically known as baryonic matter. However, this only represents a small portion of the total matter in our Universe. 

This portion equates to about 4.6% of all the matter in the Universe. There is an even larger chunk of the pie that is made up of Dark Matter.

Matter and energy distribution in the Universe. Credit: NASA.

To date, we don’t know what Dark Matter actually is made from. We know that it is there because it exerts a gravitational force on ordinary matter - a force that has been observed and measured, time and time again. 

Yet, it does not interact with anything in our Universe. So we can’t see it directly, it’s not reflected off materials, it doesn’t produce any electromagnetic radiation nor has it been confirmed in several experiments.

We know it is abundant in the Universe, as it can be indirectly observed in multiple models - such as the rotational velocities of spiral galaxies or the lensing effects that are presented when a large mass stands between our observations and a distant light source. 

And of course, by looking at the temperature and density of hot gas inside Galaxy Clusters, scientists can work out how much ‘pressure’ is being applied to that region through gravity and find the Dark Matter component, which often outweighs the visible component many times over.

Galactic Cavaties

Outlined in the research, astronomers found that the cavity in the Ophiuchus cluster was carved out of the x-ray gas through a rapid event, lasting only a few hundred million years, which dumped a gigantic amount of energy in the process. 

How much energy? About 1055 Joules! (J).  To put that number in context, every second the Sun emits about 4 x 1026 J, so the energy to create the cavity is equivalent to the total output of the Sun over the lifetime of the universe, multiplied by about sixty billion.

What was the immense energy source responsible for carving the gas cavity? Astronomers concluded that it must be due to jets of ultra-relativistic particles smashing into the gas. Such powerful jets, some spanning tens of millions of light-years, are quite common, but they are born in a tiny region about the size of our Solar System at the centre of a galaxy. 

In this tiny volume lurks a supermassive black hole, more than a billion times more massive than the Sun, circled by superheated gas moving close to the speed of light. Roaring electric currents in the gas generate powerful magnetic fields that thread the entire regions, funneling plasma along the polar axes of the spinning black hole, and accelerating the material as a thin, fine high-speed jet.

Diagram of a supermassive black hole. Credit: NASA

For the Ophiuchus cluster, the presence of such an active nuclear core, and its powerful radio jets, appears to have vanished, and astronomers concluding that the creation of the gas cavity actually impacted the flow of gas into the central black hole, shutting off activity and sending it to sleep. Once everything has calmed down, and gas is drawn in by the cluster’s gravitational pull, the central black hole might again roar back into life, and its jet might again carve its presence onto the sky.

Over the next decade, the next generation of radio telescopes will be coming on-line, including the Square Kilometre Array across Australia and South Africa. Astronomers will be ready to scour the heavens for these radio fossil relics, shining a light on our Universe’s violent past.

The paper is now available on arXiv