Nearly 200 such candidates were found

A team of six scientists has discovered the presence of a large number of what are known as giant radio galaxies (GRGs) across the universe. Such galaxies are, as the name suggests, huge, and the smallest one in this batch that has been discovered could big enough to hold 33 copies of the Milky Way placed next to each other. The galaxies have a supermassive black hole, which could be even billions of times as massive as the Sun, at their centre. Jets of charged particles are ejected from this black hole at very high speeds, close to that of light. In fact, the jets reach out to a distance even larger than the giant galaxies which host them, making the galaxy prominent when imaged with a radio telescope.

Nearly 200 new GRG candidates spread across the sky were found by the six researchers, most of whom were in institutes in Pune. “Twenty-five select galaxies are published in this work. [The] Rest will be published soon. Some are followed up for further studies with our own Indian radio telescope — the Giant Metrewave Radio Telescope (GMRT) located near Pune, India,” says Pratik Dabhade, who is a PhD student of Joydeep Bagchi of IUCAA in Pune, and is an author of the paper, published in The Monthly Notices of the Royal Astronomical Society.

In order to discover the 200 GRGs, Pratik and colleagues had to search carefully through 300 big radio images from the NRAO VLA Sky Survey, taken nearly two decades ago, From this they identified candidate GRGs and then further searched the (optical) host galaxies by poring over the literature.

What started off as a Master’s thesis problem for Pratik grew into a project with six people getting involved.

This is, however, not the first detection of a GRG by Indian astronomers. “There was the previous detection of a single GRG from India in 2015-16 using GMRT. It was special because it was found at a very long distance from us,” says Pratik in an email to The Hindu.

He also describes the significance of the discovery thus: “Since GRGs extend to Mpc [megaparsec] scales (which is almost the size of a galaxy cluster), they can be used as a probe of the medium between galaxies and clusters of galaxies. Finding them at a larger distance from us means finding them in the older universe. GRGs are very useful in understanding the growth and evolution of radio galaxies.”