A team led by Indian radio astronomers have reported the discovery of 34 new giant radio sources (GRSs) using the giant metrewave radio telescope (GMRT). Located near Khodad village, approximately 90 km north of Pune, the telescope was built and operated by the National Centre for Radio Astrophysics (NCRA) of the Tata Institute of Fundamental Research (TIFR).
From 2010 to 2012, a survey was performed using the GMRT to map the radio sky at 150 MHz, known as the TIFR GMRT Sky Survey (TGSS) covering about 90% of the sky. The team of astronomers used TIFR GMRT Sky Survey (TGSS) for their research due to its low frequencies and the sensitivity of the GMRT. The team discovered 34 giant radio sources during their research.
The team includes two PhD students, Netai Bhukta (SKBU, India) and Souvik Manik (MCC, India), and two astronomers, Sabyasachi Pal (MCC, India) and Sushanta K Mondal (SKBU, India). The discovery has been published in the Astrophysical Journal Supplement Series (ApJS) of the American Astronomical Society.
When contacted, Prof Ishwara Chandra C. H. Dean, GMRT Observatory, NCRA TIFR told The Indian Express that this was an important segment that was observationally difficult but crucial to understand the late stages of radio galaxy evolution.
“As the radio source grows large in size, the two ends get separated, making it difficult to observationally connect them as part of the same source. This is why only about 100 giant radio sources were known 20 years ago, which is a tiny fraction of the known population. Due to commissioning of new telescopes, especially at low frequencies like GMRT and LOFAR, the number has grown to a few thousand now. The current work of Netai Bhukta and team is a major contribution demonstrating that there are still many more giant sources yet to be discovered,” Prof Chandra said.
“In their research, they also found a couple of sources that defy convention that giant radio sources occur in low-density environments. They also found many of them at somewhat larger distances compared to what was known 10 years ago. Hence the current work shows that there are many more giant sources in the sky and also at farther distances,” Prof Chandra explained.
Dr Mondal told The Indian Express that the enormous size of these galaxies, and their powerful radio emissions, offer insights into the large-scale structure of the universe. “Scientists can learn more about the distribution and behaviour of matter in the cosmos. Studying these giant radio galaxies helps scientists understand the relationship between black hole activity and evolution of radio galaxies including how supermassive black holes affect their host galaxies,” he said.
According to Dr Sabyasachi Pal, associate professor and head of department of Pure and Applied Sciences, Midnapore City College, “`This is an important work to study the formation and evolution of radio galaxies because giant radio galaxies are supposed to be in the late stage of galaxy evolution. Moreover, detecting these sources is challenging because when radio jets evolve from the vicinity of the central supermassive black hole and grow to megaparsec scales, radio emissions are often not well visible by radio telescopes operating at high frequencies. Taking advantage of low-frequency radio telescopes such as GMRT, they discovered 34 new giant radio sources. They also found two such sources in a dense galaxy cluster medium, which poses a challenge to the common understanding of the formation of giant radio sources that are supposed to grow in low-density environments. Researchers have reported that the environment alone does not play a major role in the exceptionally large size of GRGs. They plan to present new GRG samples in their forthcoming articles, along with detailed physical properties based on multiwavelength observations, to unveil this mystery.”
BOX on Giant Radio Sources
The GRSs are among the largest objects in the Universe, and their enormous size and rarity puzzles astronomers as to how they grew to such a gigantic size. At the heart of GRSs lies a supermassive black hole, typically with a mass ranging from ten million to one billion times that of the Sun. Serving as the central engine, this black hole pulls in surrounding matter, which becomes ionised, creating a powerful electromagnetic force that propels the material outward to the edges,according to an official release issued by NCRA-TIFR.
The resulting jets of hot plasma produce massive lobes of radio emissions, spanning much greater distances than the visible size of the galaxy. Astronomers believe that GRSs represent the final stage of radio galaxy evolution due to their enormous sizes.
Such enormous projected lengths of GRSs make them interesting candidates for understanding the evolution of radio sources and studying the intergalactic medium that confines the lobes far from the parent galaxy. However, detecting such GRSs is challenging because the bridge emission connecting the two lobes is often not visible. Low-frequency radio surveys are better suited for identifying this population than higher-frequency radio surveys, as the aged plasma is brighter at low frequencies.
Two of the GRGs (J0843+0513 and J1138+4540) challenge the common understanding that GRGs grow in low-density environments. Researchers have said that the environment alone does not play a major role in the exceptionally large size of GRGs. They plan to present new GRG samples in their forthcoming articles, along with detailed physical properties based on multi-wavelength observations, to unveil the mystery.
Dr Mondal told The Indian Express that the enormous size of these galaxies, and their powerful radio emissions, offer insights into the large-scale structure of the universe. (File/ Representational Photo) 
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