Two cosmic beasts of the universe, giant “radio galaxies” twenty-two times the size of the Milky Way, have been discovered in a remarkably small patch of sky. These rare radio-loud galaxies that are almost exclusively large elliptical galaxies –enormous systems dubbed ‘giant radio galaxies with very luminous active galactic nuclei –have been discovered with South Africa’s powerful MeerKAT radio telescope with its unprecedented sensitivity to faint and diffuse radio light. These galaxies are thought to be amongst 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.
More Common than Thought
“We found these giant radio galaxies in a region of sky which is only about 4 times the area of the full Moon,” said astrophysicist Jacinta Delhaize, a Research Fellow at the University of Cape Town and lead author of the work. “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 percent. This means that giant radio galaxies are probably far more common than we thought!”
“These two galaxies are special because they are amongst the largest giants known, and in the top 10 percent 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,” said Matthew Prescott, also a Research Fellow at the University of the Western Cape and co-author of the work. “We suspect that many more galaxies like these should exist, because of the way we think galaxies grow and change over their lifetimes.”
Oldest Radio Galaxies
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. But why so few radio galaxies have such gigantic sizes remains something of a mystery.
Hidden From Sight
The giant 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, . MeerKat is a precursor to the Square Kilometre Array (SKA), which is due to become fully operational in the mid-2020s.
“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,” adds Delhaize.
Construction of the trans-continental SKA telescope is due to begin 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.
Both MeerKat and the SKA telescope will underscore Edwin Hubble’s observation that the history of astronomy is a history of receding horizons.
Image credit: Image credit: ESO 137-006 is a radio galaxy in the Norma galaxy cluster (Abell 3627). Norma is located at the crossing between several filaments in the Great Attractor region, which reveal hitherto unseen collimated synchrotron threads between its radio lobes.