If we had been around and could have seen the heart of the Abell 980 galaxy cluster some 260 million years ago, we might have seen something very spectacular.
The cluster’s brightest galaxy erupted from the activity of its supermassive black hole, an event that would blow massive bubbles that emit radio light into space.
The astronomers, led by Surajit Paul of Savitribai Phule Pune University in India, say these newly discovered bubbles, known as radio lobes or radio galaxies, are the oldest of their kind we’ve ever seen.
In addition, a pair of younger lobes have been found. A second team of astronomers led by Gopal Krishna of the University of Bombay in India has traced them back to the same parent galaxy, making the combined object a rare example of a double pair of lobes, suggesting that the galaxy’s supermassive black hole has exploded episodically.
Image of the cluster, with the parent galaxy shown in white and the radio lobes in red. (National Center for Radio Astrophysics)
Because radio lobes can extend millions of light years, much larger than the galaxies from which they originate, they can affect the intergalactic medium, the tenuous gas between galaxies. Studying these structures can help us better understand this environment, as well as the recurring and episodic activity of the supermassive black holes that create them.
Radio lobes are quite common in the Universe. Even the Milky Way has radio lobes. They occur when a supermassive black hole has an active phase, absorbing matter from the surrounding space.
Although most of the material falls onto the black hole, some is accelerated along the black hole’s external magnetic field lines to its poles, where it is ejected into space as two jets traveling at a significant percentage of the speed of light.
These jets penetrate into intergalactic space, where they expand into lobes that interact with the intergalactic medium. These lobes act like a synchrotron to accelerate electrons, producing radio emissions.
The problem is that they fade very quickly beyond our ability to detect them, and it’s rare to find examples beyond about 200 million years, as we see them. However, these relics can record valuable information about the conditions under which they formed.
Paul and his colleagues hypothesized that an environment that could prolong their survival is the warm, relaxed environment of a quiet, low-mass galaxy cluster.
Using the Giant Metrowave Radio Telescope in India, they searched galaxy clusters for just such an environment, and found one, in Abell 980, located about 2 billion light-years away. There, they detected faint radio structures—lobes that could date back to about 260 million years, spanning a distance of 1.2 million light years.
Next was to identify where the lobes had come from.
In a second paper, Krishna and his colleagues were able to trace it back to the cluster’s brightest galaxy. Now, it is in the center of Abell 980; however, Krishna and his team proved that he was not always in this position. Over approximately 260 million years, it migrated 250,000 light-years from the position in which it emitted the first pair of lobes.
Once in the center of the cluster, the galaxy erupted again, producing a second pair of lobes. Astronomers have so far found only a few dozen examples of galaxies that have been associated with two pairs of radio lobes, called double-double radio galaxies.
Because the parent galaxy of Abell 980’s two pairs of lobes has migrated, separating the lobes, Krishna and his team have dubbed these galaxies “separated double-double radio galaxies.” It is also even rarer than double-double radio galaxies; only two other candidates have been reported, making this discovery the most plausible example yet, the researchers say.
Future, more sensitive radio observations may yield even more examples, helping to shed light on the recurring nature of supermassive black hole explosions.
Both papers are currently in press with Astronomy & Astrophysics and Publications of the Astronomical Society of Australia, respectively. They can be found here and here.