You look for the largest objects in the universe and two come along at once: Astroboffins spot mega radio galaxies dwarfing Milky Way

These things could be more common than first thought


Astronomers have discovered the two largest-known radio galaxies to date. At a whopping 62 times the size of our own Milky Way, they are believed to be the largest single objects yet found in the universe.

Radio galaxies, characterized by their large powerful billowing jets of radio emissions from a supermassive black hole gobbling mass at their centers, are fairly common in space. Humongous ones, measuring at least 700 kiloparsecs in size – that’s about 22 times the size of the Milky Way, are much more rare.

A large team of researchers led by the University of Cape Town, South Africa, however, this month said they managed to find not one but two huge radio galaxies over a small patch of sky. “We found these giant radio galaxies in a region of sky which is only about four times the area of the full Moon,” said Jacinta Delhaize, lead author of a study into the discovery published in the Monthly Notices of the Royal Astronomical Society and a research fellow at the university.

“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 per cent." Delhaize reckons the discovery isn’t just a stroke of luck: she reckons the findings show gigantic radio galaxies are actually more common than previously believed.

Only 800 radio-jet-emitting galaxies larger than 700 kiloparsecs have been detected so far, we’re told. Despite their sheer size, they’re difficult to spot. Their large distances from Earth mean that their massive plumes of radiation are very faint. The astronomers were able to see these two objects as part of the MeerKAT International Gigahertz Tiered Extragalactic Exploration (MIGHTEE) survey, thanks to an array of radio telescopes located in the Northern Cape of South Africa.

“Until now, telescopes have not been able to detect giant radio galaxies that are both very large and very faint. This is due to technical limitations of the telescopes,” Delhaize told The Register. "The new MeerKAT telescope has amazing capabilities in this area, and so we can now start to see objects like these. The fact that we have found two in a relatively small patch of sky leads us to believe that giant radio galaxies are far more common than we previously thought."

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The radiation comes from charged particles, such as electrons, being whizzed around to nearly the speed of light by a supermassive black hole accreting mass. The particles end up getting deflected by the galaxy’s magnetic fields and are ejected into space as massive plumes of plasma flying in opposite directions.

It is the distance from one end of a jet to another that astronomers use to measure the length of a radio galaxy. “These two galaxies are special because they are amongst the largest giants known, and in the top 10 per cent of all giant radio galaxies,” said Matthew Prescott, co-author of the paper and a research fellow at the University of the Western Cape. “They are more than two 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.”

There are clusters of galaxies that dwarf these two objects in terms of size and mass, though these are made up of a collection of galaxies rather than being lone objects. Delhaize estimated that the two objects are located about 2.1 and 3.8 billion light years from the Earth.

Giant radio galaxies are believed to be older than their smaller counterparts. The radio jets have had more time to extend to greater distances. Astronomers are particularly interested in how their extreme environments affect the galaxy’s evolution and lifetime.

“We are hoping to find more of these elusive giant radio galaxies as the MIGHTEE survey progresses. This will hopefully help us to understand more about how radio galaxies evolve throughout their lifetimes and influence their host galaxies, perhaps even by ‘killing’ them by blowing out all of the gas and preventing the formation of new stars,” she concluded. ®


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