South Africa’s world-leading 64-dish radio telescope array, MeerKAT, which was inaugurated in July 2018, has registered another significant success and again increased mankind's knowledge of the cosmos. In a study published recently in the Monthly Notices of the Royal Astronomical Society, a team of three scientists from South African and British universities reported that, by using MeerKAT, they had located two previously unknown giant radio galaxies.

Significantly, the two new discoveries were both located in the same small area of the sky. Also important was the fact that previous radio telescope surveys of that part of the sky, including by the Karl G Jansky Very Large Array, in the US, and the Giant Metre-Wave Radio Telescope. in India, had failed to detect them.

“We found these giant radio galaxies in a region of the sky which is only about four times the area of the full Moon, though the galaxies are much further away and much larger than the Moon,” highlighted study lead author and University of Cape Town research fellow Dr Jacinta Delhaize. “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 extremely small. This means that giant radio galaxies are probably far more common than we thought.”

“The MeerKAT telescope is the best of its kind in the world,” affirmed study co-author and Oxford University senior researcher Dr Ian Heywood. “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.”

Radio galaxies are galaxies which are shooting forth gigantic jets of electromagnetic radiation in the radio wavelengths. These transmissions are called ‘radio light’ by astronomers. The jets are connected with supermassive black holes in the centres of these galaxies.

What happens is that when large quantities of interstellar dust and gas are attracted to the black hole, and start orbiting and falling into it, huge amounts of energy, including charged particles, are released. In some, but not all galaxies, these charged particles interact with strong magnetic fields in the proximity of the black hole, and the result is huge jets of radio light shooting out of the galactic core, pretty much perpendicular to the plane of the galaxy concerned.

These jets shoot vast distances into intergalactic space and can extend much further than the galaxy itself. But while the number of known radio galaxies runs into the many hundreds of thousands, the vast majority have jets of radio light of 700 000 parsecs or less. (A parsec is equivalent to 3.26 light years, or more than 30-trillion kilometres.) Only some 800 radio galaxies with radio light jets that exceed 700 000 parsecs (a distance which is some 22 times the size of the Milky Way) are known. These are known as giant radio galaxies.

The two giants discovered by MeerKAT have jets that are greater than two-million parsecs in length, which is about 6.5-million light years or some 62 times the size of the Milky Way. “Yet they are fainter than others of the same size,” pointed out study co-author and University of the Western Cape research fellow Dr Matthew Prescott. “We suspect that many more galaxies like these should exist, because of the way we think galaxies should grow and change over their lifetimes.”

They were discovered in new radio maps of the sky that have been created by the continuing MeerKAT International Gigahertz Tiered Extragalactic Exploration survey (acronymed to MIGHTEE). “We found large-scale radio jets coming from the central galaxies, as well as fuzzy cloud-like lobes at the ends of the jets,” reported Heywood. “We know that these galaxies are several billion light years away, and so it was the discovery of these enormous jets and lobes in the MIGHTEE map that allowed us to confidently identify the objects as giant radio galaxies.”

“The existence of the two MIGHTEE giant radio galaxies provides tantalising evidence that a large population of faint, very extended giant radio galaxies may exist,” observed Delhaize. “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. We hope to uncover more of these . . . in the MIGHTEE survey.