What are radio galaxies? Scientists discover 2 giants that dwarf the Milky Way and are 62 times its size

Among the largest giants known, the findings suggest that giant radio galaxies may be more common than previously believed


                            What are radio galaxies? Scientists discover 2 giants that dwarf the Milky Way and are 62 times its size
MGTC J095959.63+024608.6 and MGTC J100016.84+015133 were found with the powerful MeerKAT telescope (I. Heywood, Oxford/Rhodes/SARAO)

Astronomers have discovered two cosmic beasts in a small patch of sky. Dwarfing the Milky way, the two new radio galaxies are thought to be among the largest single objects in the universe. 

Whereas normal radio galaxies are fairly common, only a few hundred of these have radio jets exceeding 700 kilo-parsecs in size, or around 22 times the size of the Milky Way. The latest research shows the discoveries – named MGTC J095959.63+024608.6 and MGTC J100016.84+015133.0 – are more than 2 Mega-parsecs across: about 6.5 million light-years or about 62 times the size of the Milky Way. These “truly enormous systems” are dubbed “giant radio galaxies,” say authors.

According to scientists, the findings suggest that giant radio galaxies may be more common than previously believed. It is thought that the giants are the oldest radio galaxies, which have existed for long enough (several hundred million years) for their radio jets to grow outwards to these enormous sizes. If this is true, then many more giant radio galaxies should exist than are currently known, explain experts in their analysis published in the Monthly Notices of the Royal Astronomical Society.

Only a few hundred radio galaxies have radio jets exceeding 700 kilo-parsecs in size, or around 22 times the size of the Milky Way (Getty Images)

“These two galaxies are special because they are among the largest giants known, and in the top 10% of all giant radio galaxies. They are more than 2 Mega-parsecs across, which is around 6.5 million light-years or about 62 times the size of the Milky Way. Yet they are fainter than others of the same size. We suspect that many more galaxies like these should exist, because of the way we think galaxies grow and change over their lifetimes,” writes co-author Dr Matthew Prescott, a research fellow at the University of the Western Cape.

Lead author Dr Jacinta Delhaize, a research fellow at the University of Cape Town, believes that the work will give astronomers further clues about how galaxies have changed and evolved throughout cosmic history. “It’s also a way to understand how galaxies may continue to change and evolve – and even to work out how old radio galaxies can get. Their discovery means that a clearer understanding of the evolutionary pathways of galaxies is beginning to emerge. This is tantalizing evidence that a large population of faint, very extended giant radio galaxies may exist. This may help us understand how radio galaxies become so huge and what sort of havoc supermassive black holes can wreak on their galaxies,” emphasizes Dr Delhaize.

What are radio galaxies?

Radio galaxies get their name from the fact that they release huge beams, or ‘jets’, of radio light. These happen through the interaction between charged particles and strong magnetic fields related to supermassive black holes at the galaxies’ hearts.

According to researchers, galaxies come in many different shapes and sizes, and several galaxies have a supermassive black hole at their center. The larger the galaxy, the larger the black hole at its center. In many galaxies, they simply sit passively at the heart of the galaxy.

If a large population of faint, very extended giant radio galaxies exist, it could help researchers understand how radio galaxies become so huge and what kind of havoc supermassive black holes can wreak on their galaxies (Getty Images)

In some galaxies, however, gas and dust are falling into this supermassive black hole causing vast quantities of energy to be released. This sometimes results in energetic streams of particles – channeled by twisted magnetic fields – being ejected from the galaxy center. The release of powerful jets, extending from the galaxy’s center out into space, is a common structure for radio galaxies.

“Many galaxies have supermassive black holes in their midst. When large amounts of interstellar gas start to orbit and fall in towards the black hole, the black hole becomes ‘active’: huge amounts of energy are released from this region of the galaxy,” explains Dr Delhaize in The Conversation. She writes, “In some active galaxies, charged particles interact with the strong magnetic fields near the black hole and release huge beams, or ‘jets’, of radio light. The radio jets of these so-called ‘radio galaxies’ can be many times larger than the galaxy itself and can extend vast distances into intergalactic space.”

How was the current detection made?

The radio galaxies were spotted in new radio maps of the sky created by the MeerKAT International Gigahertz Tiered Extragalactic Exploration (MIGHTEE) survey. It is one of the large survey projects underway with South Africa’s MeerKAT radio telescope, a precursor to the Square Kilometre Array (SKA), which is due to become fully operational in the mid-2020s.

Dr Ian Heywood, a co-author at the University of Oxford, says: “The MeerKAT telescope is the best of its kind in the world. We have managed to identify these giant radio galaxies for the first time because of MeerKAT’s unprecedented sensitivity to faint and diffuse radio light.”

The MeerKAT radio telescope is a precursor to the Square Kilometre Array (SKA), which is due to become fully operational in the mid-2020s (Getty Images)

“In the past, this population of galaxies has been hidden from our ‘sight’ by the technical limitations of radio telescopes. However, it is now being revealed thanks to the impressive capabilities of the new generation of telescopes,” says Dr Delhaize.

The giants were spotted in a region of the sky which is only about 4 times the area of the full Moon. “Based on our current knowledge of the density of giant radio galaxies in the sky, the probability of finding two of them in this region is less than 0.0003%,” notes Dr Delhaize.

Construction of the SKA telescope is due to commence in South Africa and Australia in 2021 and continue until 2027. Science commissioning observations could begin as early as 2023, and it is hoped that the telescope will reveal larger populations of radio galaxies than ever before and revolutionize our understanding of galaxy evolution.

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