A team of Indian astronomers, including from Pune-based Inter University Centre for Astronomy and Astrophysics, has reported that they have detected a large number of an extremely rare kind of galaxies, called Giant Radio Galaxies, using data from a 20-year-old, highly sensitive radio survey. The researchers, whose findings have been published in Monthly Notices of the Royal Astronomical Society, are now aiming at a better understanding of the physical properties of these radio giants.
A radio galaxy is a kind of active galaxy, which in turn differs from a typical galaxy on account of its core of emission, called an active galactic nucleus. A radio galaxy is an active galaxy that emits large amounts of radio waves, powered by the galaxy’s supermassive black hole. Giant radio galaxies, or GRGs, are the largest kind, spanning 3 million light years or more — the equivalent of stacking 33 Milky Way-sized galaxies in a straight line.
GRGs are visible only to radio telescopes and hence hard to detect. The first such galaxy was found in 1974, followed by about 300 more since then. The team at IUCAA has identified several hundred candidates, and confirmed the first 25 as radio galaxies, some of them up to 10 million light years across. “Our work will help in understanding how these galaxies grow to be so large,” said lead author Pratik Dabhade, a junior research fellow under the Indo-French Centre for the Promotion of Advanced Research project at IUCAA and a PhD student at the Observatory of Leiden, Netherlands.
“We are studying whether they are born in regions of very sparse galaxy density or they have extremely powerful and long-lasting radio jets which allow them to expand to very huge distances,” Dabhade told The Indian Express from the Netherlands. Researchers have been working on finding and studying rare species of radio galaxies for insights into how these galaxies produce energy at their centres and transfer it to the medium outside the galaxy.
The team of Indian scientists includes Joydeep Bagchi (IUCAA) Madhuri Gaikwad (Max Planck Institute, Bonn), Shishir Sankhyayan (IISER, Pune), Mamta Pommier (CNRS Observatoire de Lyon) and Somak Raychaudhury (IUCAA director). “The astronomers benefited from the generous support from the Indo-French Centre for the Promotion of Advanced Research,” Dabhade said. It has been working with a set of 300 large images from the US National Radio Astronomy Observatory’ Very Large Array survey more than 20 years ago. They inspected these images over almost two years to identify many giant radio galaxies.
“After finding them in radio images, we have to find their counterparts in the optical domain,” Dabadhe said. These have to be followed by comparison, and deeper observations with other radio telescopes such as the Giant Metrewave Radio Telescope near Pune, to confirm the GRGS’ existence. “When we point a normal (optical) telescope at these galaxies, we image the starlight from them,” Raychaudhury described the search. “However, telescopes that can observe other forms of radiation and energy, such as radio waves and X-ray radiation, can image components of these galaxies other than stars… Radio telescopes can image the really fast-moving charged particles in a galaxy, which are associated with high energy outflow at the centres of galaxies where a supermassive black hole exists. These emit a form of radiation known as synchrotron radiation, which can be seen by radio telescopes.”
These particles, mostly electrons, move at speeds close to the speed of light and show up as thin jets shooting out of the centre of the galaxy. These outflows last for millions of years, during which these particles reach outside the galaxy, culminating into lobes as they lose energy. In most cases these jets are the same size as the galaxies themselves. There are two big mysteries about GRGs: Why they are so big, and why they are apparently rare.
Discovering more GRGs, the astronomers said, will allow them to study them with other multi-wavelength telescopes to find out their age, central black hole mass, star formation rate etc. Among the answers scientists are looking for are: Are these galaxies in which the black holes emit more energy than in others? Are these galaxies sitting in an under-dense environment such that the jet travel to larger distances? Do the energetic particles give rise to other physical effects by themselves that make them longer-lived and hence enable them to travel very far?