There are many different types of telescopes. Some can detect optical light, which is the type of light we see with our eyes. Others can detect types of light that are invisible to our eyes, such as X-ray, UV, infrared or radio. These telescopes all look quite different to one another. For example, a radio telescope is usually made up of one or more metal “dishes”, which look similar to satellite dishes. Each of these different telescopes provides us with different information about objects in space, such as galaxies. The stars inside a galaxy can be seen with optical telescopes, while the dust can be seen with infrared telescopes. But there are other weird parts of galaxies that can only be seen with radio telescopes.
Most galaxies, like our own Milky Way, have a supermassive black hole at the centre. It can be millions of times the mass of our Sun. In radio galaxies, “stuff” is falling into the supermassive black hole. It’s gorging itself on gas and dust and generally wreaking havoc. As this happens, enormous amounts of energy are released and fast-moving particles like electrons can get caught up in the twisted magnetic fields close to the black hole. These get funnelled out into space in huge plasma jets which grow out of either side of the galaxy.Think of a water jet from the blowhole of a whale. It comes out in a narrow column then turns into a cloudy plume. That’s more or less what the plasma jets look like. The plasma jets glow in radio light, which can only be seen with radio telescopes. This is why these objects are called “radio galaxies” and the jets are called “radio jets”. The radio jets can extend out to enormous distances from the centre of the galaxy. When they grow to larger than 700 kiloparsecs from end-to-end, they are called “giant radio galaxies”, or GRGs. These are the largest single objects in the Universe and are more than 22 times the size of the Milky Way!
One kiloparsec (kpc) is equal to 1 000 parsecs, or 3 260 light years. Our Milky Way is about 30 kpc across and the nearest galaxy, the Canis Major dwarf, is located about 13 kpc from the centre of our Milky Way.
Radio galaxies, especially GRGs, are important because they help us to understand how galaxies have changed throughout the cosmic history of the Universe. The plasma jets can heat up the gas in the rest of the galaxy, or completely blow it out of the galaxy. This essentially “kills” the galaxy because without cold gas, new stars cannot form. Studying the way the jets change the galaxy helps us understand why galaxies look the way they do today, and why they looked different in the past.
However, these giants can sometimes be difficult to find. They often appear faint and fuzzy, in a way that is difficult even for radio telescopes to see. Thankfully, South Africa has a powerful new radio telescope called MeerKAT which is particularly good at finding these kinds of things. We have been using MeerKAT to study radio galaxies through a project called the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE). The project is well-named because with it we have found two of the mightiest giant radio galaxies known! They are more than 65 times the diameter of the Milky Way!
It was incredibly exciting to find these two mega-giants because it means there are probably many more out there in the cosmos just waiting for us to find. Our discovery demonstrates the amazing abilities of MeerKAT and we think it will be able to reveal even more of the Universe’s secrets!
Though MeerKAT is already making amazing discoveries, it will soon be incorporated into an even bigger radio telescope - a mega-science project called the Square Kilometre Array (SKA). This enormous telescope will span two continents (Africa and Australia) and be so powerful that it is expected to revolutionise our understanding of radio galaxies and the way galaxies evolve. We can’t wait to see what it will discover!
Written by: Dr Jacinta Delhaize: SARAO Postdoctoral Fellow at the UCT Department of Astronomy
* Main Image: South Africa's MeerKAT telescope. Credit: South African Radio Astronomy Observatory (SARAO)