Radio images of the sky have revealed hundreds of ‘baby’ and supermassive black holes in distant galaxies, with the light from the galaxies bouncing around in unexpected ways.
Galaxies are enormous cosmic bodies, tens of thousands of light years in size, made up of gas, dust and stars (like our sun).
Given their size, you would expect that the amount of light emitted from galaxies would change slowly and steadily, over time scales well beyond a person’s life.
But our research, published in the Monthly Communications from the Royal Astronomical Society, found a surprising population of galaxies whose light changes much faster, in just a matter of years.
What is a radio system?
Astronomers believe there is a supermassive black hole at the center of most galaxies. Some of these are ‘active’, which means that they emit a lot of radiation.
Their powerful gravitational fields take matter from their environment and tear it apart into a revolving ring of hot plasma called an “accretion disk.”
This disk orbits the black hole at almost the speed of light. Magnetic fields accelerate high-energy particles from the disk in long, thin streams or ‘jets’ along the black hole’s rotational axes. As they move further from the black hole, these jets blossom into large mushroom-shaped clouds or ‘lobes’.
This whole structure is what makes up a radio system, so called because it gives off a lot of radio frequency radiation. It can be hundreds, thousands, or even millions of light years across and therefore it can take eons before dramatic changes are visible.
Astronomers have long wondered why some radio galaxies have huge lobes, while others remain small and limited. There are two theories. One is that dense material around the black hole stops the jets, often referred to as frustrated lobes.
However, the details surrounding this phenomenon remain unknown. It is still unclear whether the lobes are only temporarily confined by a small, extremely dense environment – or whether they are moving slowly through a larger but less dense environment.
The second theory to explain smaller lobes is that the jets are young and have not yet stretched great distances.
The supermassive black hole of Hercules A emits high-energy particle jets in radio labs. (NASA / ESA / NRAO)
Old ones are red, babies are blue
Both young and old radio galaxies can be identified through a clever use of modern radio astronomy: looking at their ‘radio color’.
We looked at data from the GaLactic and Extragalactic All Sky MWA (GLEAM) survey, which sees the sky at 20 different radio frequencies, giving astronomers an unparalleled ‘radio color’ image of the sky.
The data shows that baby radio systems appear blue, which means that they are brighter at higher radio frequencies. Meanwhile, the old and dying radio galaxies look red and are brighter in the lower radio frequencies.
We identified 554 baby radio sets. When we looked at identical data a year later, we were surprised to see 123 of these bouncing around in their brightness and seemed to flicker. This gave us a puzzle.
Just over a light-year in size cannot vary that much in brightness in less than a year without violating the laws of physics. So our galaxies were much smaller than expected, or something else happened.
Fortunately, we had the data we needed to find out.
Previous research on radio galaxy variability has either used a small number of galaxies, collected archival data from many different telescopes, or has been conducted with only a single frequency.
For our study, we examined more than 21,000 galaxies at multiple radio frequencies in one year. This makes it the first ‘spectral variability’ study, allowing us to see how galaxies change brightness at different frequencies.
Some of our bouncing baby radio galaxies have changed so much over the year that we doubt whether they are babies at all. There is a chance that these compact radio galaxies are in fact anxious teens growing into adults quickly, much faster than we expected.
While most of our variable galaxies increased or decreased in brightness in all radio colors about the same amount, some did not. 51 galaxies also changed brightness in both and color, which may indicate the cause of the variability.
Artist’s impression of SKA-mid (left) and SKA-low (right) telescopes. (SKAO / ICRAR / SARAO)
Three possibilities for what is happening
1) Twinkling galaxies
When light from stars travels through the Earth’s atmosphere, it becomes distorted. This creates the twinkling effect of stars we see in the night sky, called ‘scintillation’. The light from the radio galaxies in this study passed through our Milky Way Galaxy to reach our telescopes on Earth.
So the gas and dust in our galaxy could have distorted it in the same way, resulting in a twinkling effect.
2) Watch the loop
In our three-dimensional universe, black holes sometimes shoot high-energy particles directly at us on Earth. These radio systems are called ‘blazars’.
Instead of seeing long thin rays and large mushroom-shaped lobes, we see blazars as a very small point of light. They can exhibit extreme variability in short timescales, as every tiny release of matter from the supermassive black hole itself is aimed right at us.
3) Farmers with a black hole
When the central supermassive black hole burps some extra particles, they form a clump that slowly travels past the jets. As the clump propagates outward, we can detect it first in the ‘radio blue’ and later in the ‘radio red’.
So we can detect giant black hole farmers slowly traveling through space.
where to now?
This is the first time that we have the technological capability to conduct a large-scale variability study across multiple radio colors. The results suggest that our understanding of the radio sky is lacking, and perhaps radio galaxies are more dynamic than we expected.
With the next generation of telescopes coming online, especially the Square Kilometer Array (SKA), astronomers will build a dynamic image of the sky over many years.
In the meantime, it’s worth taking a look at these oddly behaving radio galaxies and keeping a close eye on the bouncing babies too. 
Kathryn Ross, PhD Student, Curtin University and Natasha Hurley-Walker, Radio Astronomer, Curtin University.
This article has been republished from The Conversation under a Creative Commons license. Read the original article.