Radio images from the sky have revealed hundreds of ‘baby’ and supermassive black holes in distant galaxies, with the galaxies reflecting in unexpected ways.
Galaxies are vast 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 the amount of light emitted from galaxies to change slowly and steadily, over time scales far beyond a person’s lifetime.
But our research, published in the Monthly notices from the Royal Astronomical Society, found within a few years a surprising population of galaxies whose light changes much faster.
What is a radio system?
Astronomers think there is a supermassive black hole in the center of most galaxies. Some of these are ‘active’, which means they emit a lot of radiation.
Their powerful gravitational fields draw matter from their environment and tear it apart into a hot plasma orbit called a ‘growth disk’.
This disk revolves almost around the black hole at the speed of light. Magnetic fields accelerate high-energy particles from the disk in long, thin currents or ‘rays’ along the axis of rotation of the black hole. As they get further from the black hole, these rays bloom in large mushroom-shaped clouds or ‘lobes’.
This whole structure forms the radio system, supposedly because it emits a lot of radio frequency radiation. It can take hundreds, thousands or even millions of light years, and so it can take centuries to show dramatic changes.
Astronomers have long questioned why some radio systems house enormous lobes, while others remain small and limited. There are two theories. One is that the rays are held back by dense material around the black hole, often called 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 that they are slowly pushed through a larger but less dense environment.
The second theory to explain smaller lobes is that the rays are young and have not yet reached great distances.
Hercules A’s supermassive black hole that emits high-energy particle rays in radiolobes. (NASA / ESA / NRAO)
Old people are red, babies are blue
Both young and old radio systems can be identified by a clever use of modern radio astronomy: look at their ‘radio color’.
We looked at data from the GaLactic and Extragalactic All Sky MWA (GLEAM) survey, which see the sky at 20 different radio frequencies, giving astronomers an unparalleled ‘radio color’ view of the sky.
From the data, baby radio systems appear blue, meaning they are brighter at higher radio frequencies. Meanwhile, the old and dying radio systems look red and are brighter in the lower radio frequencies.
We identified 554 baby radio systems. When we looked at identical data a year later, we were surprised to see that 123 of these were bouncing around in their brightness and looking like they were flickering. This left us with a mystery.
Something that is more than one light-year in size cannot change in brightness for less than one year without violating the laws of physics. So, our galaxies were much smaller than expected, or something else happened.
Luckily we had the necessary data to find out.
Previous research on the volatility of radio systems has either used a small number of galaxies, archived data collected from many different telescopes, or done with only a single frequency.
For our research, we examined more than 21,000 galaxies over multiple years over multiple radio frequencies. This makes it the first ‘spectral variability’ survey, which enables us to see how galaxies change brightness at different frequencies.
Some of our reflective radio radio galaxies have changed so much over the years, we doubt if they are babies at all. There is a chance that these compact radio systems are actually becoming anxious teenagers who are rapidly growing into adults faster than we expected.
While most of our variable galaxies in all the radio colors increased or decreased by about the same amount, some did not. 51 galaxies also changed brightness in both and color, which may be an idea of what causes the variability.
Artist’s impression of SKA middle (left) and SKA low (right) telescopes. (SKAO / ICRAR / SARAO)
Three possibilities for what happens
1) Sparkling galaxies
As light from stars moves through the earth’s atmosphere, it is distorted. This creates the sparkling effect of stars we see in the night sky, called ‘glitter’. The light from the radio systems in this recording passed through our Milky Way system to reach our telescopes on earth.
Thus, the gas and dust in our galaxy could have distorted it in the same way, resulting in a sparkling effect.
2) Look down the barrel
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 jackets as a very small bright dot. They can be extremely volatile in short time scales, as any small ejection of matter from the supermassive black hole itself is aimed directly at us.
3) Black hole bars
As the central supermassive black hole ‘drills up’ a few extra particles, it forms a clump that moves slowly along the rays. As the clump spreads outwards, we can detect it first in the ‘radio blue’ and then later in the ‘radio red’.
We can therefore detect giant black holes moving slowly through space.
Where to now?
This is the first time we have the technological capability to conduct a large-scale variability survey across multiple radio colors. The results suggest that we lack understanding of the radio sky, and perhaps radio systems are more dynamic than we expected.
As the next generation of telescopes comes online, especially the Square Kilometer Array (SKA), astronomers will build a dynamic image of the sky over many years.
Meanwhile, it’s worth watching these strangely performing radio systems and also keeping a close eye on the reflective babies. 
Kathryn Ross, PhD student, Curtin University and Natasha Hurley-Walker, radio astronomer, Curtin University.
This article was published from The Conversation under a Creative Commons license. Read the original article.