Imagine you are playing soccer on a large grass field. The game ends and you take a break, and then you notice that the coin you had in your pocket has fallen somewhere. If you set out to try to find it on your own, it will take you a long time and you probably won’t even succeed. But if you recruit all your friends for the task and split up into different parts of the field, you’ll find it pretty quickly.
That’s the analogy University of Mumbai professor Ananda Hota gives to his Facebook group as they scan the sky together for rare celestial objects.
Dr. Hota and his staff have been leading the RAD@home group since 2013. Today, it has about 4,700 members. Most of them are not professional astronomers, but they play an important role in making real astronomical discoveries.
So, on October 2, the group informed a a very unusual object first identified only in 2019—the odd-numbered radio circle (ORC)—using data from the LOFAR network of telescopes in Europe. ORCs are very large but very faint round radio sources that usually surround a distant galaxy. Prevailing theories suggest that ORCs are the remnants of merging supermassive black holes or massive galactic shock waves, and are among the least studied objects in deep space.
In addition to this headline discovery, the team is regularly engaged in excavations significant information about new galaxies and transient astronomical phenomena. As such, RAD@home demonstrates the power of citizen science-led research with the ably assisted by one of the world’s most powerful radio telescopes, the Giant Metrewave Radio Telescope (GMRT) near Pune.
It comes full circle
Depending on their shape and structure, galaxies come in one of four main types: spiral, elliptical, irregular, and lenticular. Spiral galaxies like the Milky Way, with their characteristic winding arms, contain many hotter, younger, bluer stars – while elliptical galaxies, which tend to be more elongated, are dominated by older, cooler, redder stars.
At the center of most massive galaxies is a supermassive black hole, the mass of which is millions and billions of times greater than the mass of our sun. And although in most galaxies these monsters are quiet, in some they are extremely active. They feed on gas, dust and other debris that surrounds them, releasing huge amounts of energy. Such galaxies are called active. And when their black holes launch jets of plasma that glow brightly at radio frequencies, they are called radio galaxies.
These jets can extend for millions of light years on either side of the galactic plane. At the ends of these jets are two broad “radio bands”. The appearance is not unlike two balloons tied with thin strings on either side of the sphere.
Because these jets usually form in massive elliptical galaxies, astronomers have long believed that spiral galaxies cannot host them. This assumption was overturned when I want to and others. discovered an exception during his postdoc: a rare case of a spiral galaxy producing large radio petals.
“It was an accidental discovery,” he said.
It was 2011 and the Internet was starting to seep into everyday life through social media. Citizen science projects, such as Zuniversom, gained momentum thanks to their scientific discoveries. When Dr. Hota shared news of his discovery on social media, he was surprised by the questions and comments his post generated.
“When you’re doing science, it becomes so difficult for the average person to understand that we astronomers sometimes feel almost useless to the public,” Dr. Hota said.
His own interest in science and astronomy developed in high school when he listened to radio broadcasts and read about galaxies, black holes and powerful telescopes.
“It’s time to give back,” he said, so he created a Facebook group and invited students to join, learn about astronomy and contribute to research.
Rare things
Each search begins with virtual lectures over the weekend, where Dr. Hota and other researchers teach participants how to recognize the standard color and structure of galaxies in ultraviolet, optical, infrared and radio images.
Radio galaxies can be classified by shape and brightness. In the widely used Fanarov–Riley (FR) classification, FR I sources are less bright, with jets that fade as they move outward, while FR II sources are more powerful, with bright hot spots at the ends of their petals. Astronomers also identify special subtypes, such as X-shaped, double, or giant radio galaxies, each of which shows distinct episodes of jet activity.
Once participants understand what a typical radio galaxy looks like, they are encouraged to look for sources that meet expectations.
“Anything that looks dim, fuzzy and irregular in the data is a sign of past black hole activity,” Dr Hota said.
Their latest discovery, a rare “double ORC,” was published months after Prasun Machado, a RAD@home student, spotted two faint round structures in an unusual radio galaxy in the LOFAR data. These circles, much larger than the galaxies themselves, turned out to be an ORC pair, only the second known instance of such a twin. It was soon found to be one of the most distant and powerful ORCs ever recorded.
“When you find something very rare or very different from the norm, you suddenly have the opportunity to start a new investigation into the unknown,” Dr. Hota said.
Over the following months, Dr. Hota and his colleagues investigated the find further using archival data from various radio and optical telescopes.
Any astronomer
There is still no generally accepted definition of ORC. Their true nature remains uncertain, and astronomers are exploring several possibilities.
Dr. Hota said one idea is that when galaxies collide, they can create powerful shock waves that propagate outward into intergalactic space. Over a billion years, these waves could form large circular structures visible only in radio waves. Another possibility is that ORCs are the result of massive explosions, perhaps when two supermassive black holes merge.
In the case of twin ORCs, Dr. Hota suggested that the plasma rings may expand in opposite directions, forming two large circles located on either side of the galaxy.
“We need to discover and characterize many more such objects,” said Dr. Hota. “Only then can we begin to understand their true nature.”
For now, he and his colleagues are looking to take advantage of the vast amount of data collected by the world-class GMRT facility, which is one of the largest and most sensitive low-frequency radio telescopes in the world.
“Our own GMRT is free for anyone to use, but this capacity is under-utilised,” believes Dr Hota. “People still think that education and research are two separate stages: first you learn, then you do research. That model is over. You can join research at any stage of your career if you find a good mentor and a good project. Once we create this combined model of learning and discovery through various citizen science projects, Indian astronomy will grow faster.”
Monica Mondal is a freelance science and environmental journalist.
