Our universe may be filled with unseen giants. Astronomers have discovered two giant radio galaxies, which are some of the largest-known objects in the universe. This revelation suggests that the enormous galaxies may be more common than previously believed.
The study published Monday in the Monthly Notices of the Royal Astronomical Society.
Astronomers found the two galaxies in new radio maps that were created using the MeerKAT radio telescope in South Africa.
Radio galaxies, which can be quite common, are brightest in radio wavelengths of light. Centaurus A is a well-known radio galaxy to astronomers. It's the fifth brightest galaxy in the sky and is about 12 million light-years from Earth.
Centaurus A features a supermassive black hole at its center, which releases powerful jets that flow perpendicularly to the galaxy's disk -- a common structure for radio galaxies.
Giant radio galaxies, however, are more rare and have jets that exceed 22 times the size of our Milky Way galaxy. The Milky Way is a barred spiral galaxy, which is also fairly common, and looks like it sounds: a spiral-shaped galaxy with a central bar-shaped structure made of stars.
Now, astronomers have found two giant radio galaxies in a rather small patch of sky.
"We found these giant radio galaxies in a region of sky which is only about 4 times the area of the full Moon," said Jacinta Delhaize, lead study author and research fellow at the University of Cape Town, in a statement.
"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. This means that giant radio galaxies are probably far more common than we thought!"
Just because the galaxies are massive doesn't mean they're the easiest to spot, which is why astronomers didn't observe them until now.
"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, study coauthor and research fellow at the University of the Western Cape in South Africa, in a statement.
"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."
Astronomers still don't quite understand why only some radio galaxies grow to this massive size. Some believe that the giant radio galaxies are the oldest, which has allowed their radio jets to increase outward over several hundred million years.
If that's the case, astronomers think there should be more of them in the universe than those they have recorded.
The MeerKAT telescope, located in the Karoo region of South Africa, includes an array of 64 radio dishes and has been operational since July 2018. The powerful telescope is sensitive to faint radio light, which allowed researchers to find the giant radio galaxies.
The telescope array is a precursor to an upcoming telescope, the transcontinental Square Kilometer Array, which will begin construction in both South Africa and Australia this year and begin observations in the mid-2020s.
The array will include thousands of dishes and up to a million low-frequency antennas in an effort to build the world's largest radio telescope. Despite the fact that these dishes and antennas will be in two different parts of the world, together they will create one telescope that has over 1 million square meters of collecting area, meaning that astronomers can survey the entire sky much more quickly than with other telescopes.
It will also exceed the image resolution of the Hubble Space Telescope and image large portions of the sky in sensitive detail.
The Square Kilometer Array may discover more radio galaxies and shed light on how galaxies evolve.
"In the past, this population of galaxies has been hidden from our 'sight' by the technical limitations of radio telescopes," Delhaize said. "However, it is now being revealed thanks to the impressive capabilities of the new generation of telescopes."
