Radio images of the sky revealed hundreds of “baby” and supermassive black holes in distant galaxies, the light of galaxies bouncing around it in unexpected ways.
Galaxies are vast cosmic bodies, with dimensions of tens of thousands of light years, made up of gas, dust and stars (like our Sun).
Given their size, you would expect the amount of light emitted by galaxies to change slowly and constantly, over periods of time well over a person’s life.
But our research, published in Monthly notifications of the Royal Astronomical Society, found a surprising population of galaxies whose light changes much faster in just a few years.
What is a radio galaxy?
Astronomers believe that there is a supermassive black hole in the center of most galaxies. Some of them are “active”, which means that they emit a lot of radiation.
Their strong gravitational fields extract matter from their surroundings and tear it into an orbiting hot plasma donut called an “accumulation disk.”
This disk orbits the black hole at almost the speed of light. Magnetic fields accelerate high-energy particles on the disk in long, thin streams or “jets” along the axes of rotation of the black hole. As they move farther away from the black hole, these jets bloom in large mushroom-shaped clouds or “lobes.”
The whole structure is what makes up a radio galaxy, so-called because it emits a lot of radio frequency radiation. It can be hundreds, thousands or even millions of light years away, and therefore it can take eons to show any dramatic changes.
Astronomers have long wondered why some radio galaxies host huge lobes, while others remain small and small. There are two theories. One is that the jets are held back by dense material around the black hole, often called frustrated lobes.
However, the details surrounding this phenomenon remain unknown. It is not yet clear whether the lobes are temporarily limited only by an extremely dense environment – or whether they push slowly through a larger but less dense environment.
The second theory that explains smaller lobes is that the jets are young and have not yet spread over long distances.
Hercules A’s supermassive black hole emits jets of high-energy particles into radio lobes. (NASA / ESA / NRAO)
The old ones are red, the babies are blue
Both young and old radio galaxies can be identified through an intelligent use of modern radio astronomy: looking at their “radio color”.
We analyzed data from the GaLactic and Extragalactic All Sky MWA (GLEAM) survey, which sees the sky at 20 different radio frequencies, giving astronomers an unparalleled view of the “radio color” of the sky.
From the data, children’s radio galaxies appear in blue, which means they are brighter at higher radio frequencies. Meanwhile, old and dying radio galaxies look red and are brighter in lower radio frequencies.
We have identified 554 radio galaxies for children. When we looked at identical data taken a year later, we were surprised to see that 123 of them were hovering around their brightness, seeming to flicker. This left us with a puzzle.
A little more than a light year in size cannot vary so much in brightness in less than a year without breaking the laws of physics. So either our galaxies were much smaller than expected, or something else was happening.
Fortunately, I had the data to find out.
Previous research on the variability of radio galaxies used either a small number of galaxies, archive data collected from many different telescopes, or was conducted using a single frequency.
For our research, we studied over 21,000 galaxies over the course of a year on multiple radio frequencies. This makes it the first “spectral variability” survey, allowing us to see how galaxies change brightness at different frequencies.
Some of our children’s radio galaxies that have changed have changed so much over the years, we doubt they are babies. Chances are that these compact radio galaxies are actually scary teens that grow rapidly in adults much faster than we expected.
While most of our variable galaxies have increased or decreased in brightness by about the same amount in all radio colors, some have not. Also, 51 galaxies changed in both brightnesses and color, which may be an indication of what is causing the variability.
The artist’s impression of 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 sparkling effect of the stars we see in the night sky, called “scintillations”. The light from the radio galaxies in this survey passed through our Milky Way galaxy 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 brilliant effect.
2) Looking down the barrel
In our three-dimensional Universe, black holes sometimes draw high-energy particles directly at us on Earth. These radio galaxies are called “blazars”.
Instead of seeing long thin jets and large mushroom-shaped lobes, we see the blazers as a very small point of light. They can show extreme variability in short periods of time, because any small ejection of matter from the supermassive black hole itself is directed directly at us.
3) Black hole bubbles
When the central supermassive black hole “overturns” some additional particles, they form an agglomeration that travels slowly along the jets. As the mass propagates outward, we can detect it first in “radio blue” and then later in “radio red.”
So it is possible to detect huge burps of black holes that travel slowly through space.
Where to now?
This is the first time we have the technological capability to conduct a large-scale multi-color radio variability survey. The results suggest that our understanding of the radio sky is lacking and that radio galaxies are more dynamic than we expected.
As the next generation of telescopes becomes 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 watching these weirdly behaving radio galaxies and keeping a close eye on babies who get up. 
Kathryn Ross, PhD student, Curtin University and Natasha Hurley-Walker, radio astronomer, Curtin University.
This article is republished from Conversation under a Creative Commons license. Read the original article.