Radio images of the sky have revealed hundreds of “baby” and supermassive black holes in distant galaxies, with galaxy light bouncing unexpectedly.
Galaxies are large cosmic bodies, tens of thousands of light-years away, made up of gas, dust, and stars (like our Sun).
Given their size, the amount of light emitted from galaxies is expected to change slowly and steadily, over periods of time far greater than a person’s life.
But our research, published in Monthly notices from the Royal Astronomical Society, found an astonishing population of galaxies whose light changes much faster, in a matter of years.
What is a Galaxy Radio?
Astronomers think there is a supermassive black hole in the center of most galaxies. Some of these are “active,” meaning they emit a lot of radiation.
Its powerful gravitational fields extract matter from its environment and tear it into an orbiting donut of hot plasma called an “accretion disk.”
This disk orbits the black hole at almost the speed of light. Magnetic fields accelerate the high-energy particles in the disk in thin or long currents or “jets” along the axes of rotation of the black hole. As they move away from the black hole, these jets bloom into large clouds or mushroom-shaped “lobes”.
This whole structure is what constitutes a galaxy radio, so called because it emits a lot of radio frequency radiation. It can be hundreds, thousands, or even millions of light-years in diameter, and therefore it can take eons to show any dramatic change.
Astronomers have long wondered why some radio galaxies host huge lobes, while others remain small and confined. There are two theories. One is that the rays are retained by a 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 surrounding environment, or whether they are slowly pushing through a larger but less dense environment.
The second theory that explains the smaller lobes is that the jets are young and have not yet spread over long distances.
The supermassive black hole of Hercules A that emits jets of high-energy particles into the radio lobes. (NASA / ESA / NRAO)
The old ones are red, the babies are blue
Both fast young and old galaxies can be identified through intelligent use of modern radio astronomy: by 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 a “radio-colored” radio-free view of the sky.
From the data, children’s radio galaxies appear blue, meaning they are brighter at higher radio frequencies. Meanwhile, the old, dying radio galaxies look red and are brighter at lower radio frequencies.
We identified 554 children’s radio galaxies. When we examined identical data taken a year later, we were surprised to see that 123 of them bounced off their brightness and seemed to flicker. This left us with a puzzle.
A little over a light year in size cannot vary so much in brightness for less than a year without breaking the laws of physics. So either our galaxies were much smaller than expected, or something else was happening.
Luckily, we had the data we needed to find out.
Previous research on the variability of radio galaxies has used a small number of galaxies, archival data collected from many different telescopes, or was conducted using only a single frequency.
For our research, we have studied more than 21,000 galaxies over a year across multiple radio frequencies. This makes it the first study of “spectral variability,” which allows us to see how galaxies change brightness at different frequencies.
Some of our radio galaxies for babies that jumped changed so much during the year, we doubt they are babies. There is a possibility that these fast compact galaxies are distressed teens that grow rapidly in adults much faster than we expected.
While most of our variable galaxies increased or decreased brightness by about the same amount in all colors of the radio, some did not. In addition, 51 galaxies changed with their brightness i color, which can be a clue as to what causes variability.
Artist’s print on the SKA-mid (left) and SKA-low (right) telescopes. (SKAO / ICRAR / SARAO)
Three possibilities for what is happening
1) Sparkling galaxies
As the light from the stars travels through the Earth’s atmosphere, it is distorted. This creates the twinkle effect of the stars we see in the night sky, called “twinkle”. The light from the radio galaxies in this survey passed through our Milky Way galaxy to reach our Earth telescopes.
Thus, the gas and dust inside our galaxy could have distorted it in the same way, resulting in a flickering effect.
2) Looking down the barrel
In our three-dimensional Universe, black holes sometimes shoot high-energy particles directly at the Earth. These radio galaxies are called “blazars”.
Instead of seeing long, thin rays and large mushroom-shaped lobes, we see the blazars as a very small, bright spot. They can show extreme variability on short time scales, as any small expulsion of matter from the same supermassive black hole is directed at us.
3) Black hole routes
When the central supermassive black hole “pulls out” some additional particles, they form a group that travels slowly along the jets. As the thickness propagates outward, we can detect it first in the “blue radius” and then in the “red radius.”
Therefore, we may be detecting belching of giant black holes that travel slowly through space.
Where now?
This is the first time we have the technological capability to conduct a large-scale variability survey on various radio colors. The results suggest that our understanding of radio sky is lacking and perhaps radio galaxies are more dynamic than we expected.
As the next generation of telescopes, particularly the Square Kilometer (SKA), is connected, astronomers will be creating a dynamic image of the sky over many years.
In the meantime, it’s worth watching these weird-behaving radio galaxies and especially watching the babies bouncing. 
Kathryn Ross, PhD student, Curtin University and Natasha Hurley-Walker, radio astronomer, Curtin University.
This article is republished from The Conversation under a Creative Commons license. Read the original article.