A six-member team of Indian astronomers led by Pratik Dabhade from Pune’s Inter-University Centre for Astronomy and Astrophysics (IUCAA) has reported the discovery of 25 extremely rare kind of galaxies called giant radio galaxies (GRGs), known to be the largest known galaxies in the universe.
In the last six decades of radio astronomy research, thousands of radio galaxies have been detected, of which only a meagre 300 are classified as GRGs.
The reason behind their large size and rarity, however, is still a mystery.
The first giant radio galaxy was discovered in the 1970s using the Westerbork Synthesis Radio Telescope in the Netherlands in 1974. We’ve come a long way since. Today, all major radio telescopes and powerful computer simulations are used to uncover the nature of these GRGs.
The six-member team under Pratik Dabhade includes Joydeep Bagchi (IUCAA), Mamta Pommier (CNRS Observatoire de Lyon), Madhuri Gaikwad (NCRA-TIFR Pune and Max-Planck Institute Bonn) and Shishir Sankhyayan (IISER Pune) and Somak Raychaudhury (IUCAA).
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The team carried out a systematic search after examining over 300 pictures in a nearly 20-year-old radio survey to discover these GRGs. The scanning took over a year and a half and was followed with rigorous research on confirming the findings at Giant Metrewave Radio Telescope (GMRT) near Pune. The discovery was published in a paper in the journal Monthly Notices of the Royal Astronomical Society.
“The huge size of GRGs has defied any theoretical explanation so far. Our work will help in understanding how these galaxies grow to be so large,” said Pratik, lead researcher at Inter-University Centre for Astronomy and Astrophysics (IUCAA, Pune) and the Netherlands’ Leiden Observatory.
“We are studying whether they are born in regions of very sparse galaxy density, or they have extremely powerful, well-collimated, long-lasting radio jets which allow them to expand to very huge distances,” he added.
According to a Hindustan Times report, these extremely active form of galaxies harbour a supermassive black hole ‘central-engine’ at the nucleus, which ejects a pair of high energy particle jets nearly at the speed of light, which terminate into two giant radio lobes.
Astronomer Joydeep Bagchi from the team said, “Understanding the life-cycle of the black hole’s energetic activity, properties of the matter which falls into it, and the influence of the surrounding medium which acts on the lobes far away from the host galaxy, and provides a ‘working-surface’ for the radio jets to act, are among the most important problems in this field”.
GRGs are visible only to radio telescopes.
According to a Times of India report, these giant radio galaxies’ size ranges from 0.8-4.0 Mpc (1Mpc is equal to three and a quarter million light-years). Most of these newly found GRGs are highly unusual, showing very powerful radio jets, feeding large, diffuse radio lobes. These giants span nearly 3 million light years across and host supermassive black holes. Their size corresponds to stacking nearly 33 Milky Way-like galaxies in a line.
Known for their unbelievably large sizes, the GRGs are believed to be the last stop of radio galaxy evolution.