Four Bengali astronomers discovered 34 new giant radio sources – GetBengal story

Four Bengali radio astronomers have recently discovered 34 new giant radio sources (GRSs) using the Giant Metrewave Radio Telescope (GMRT). This discovery, published in the ‘Astrophysical Journal Supplement Series’ (ApJS) of the American Astronomical Society, marks a significant contribution to the field of radio astronomy. Historically, only about 100 GRSs existed 20 years ago. The number has gradually increased due to advancements in low-frequency radio telescopes like GMRT and LOFAR, but the discovery of 34 new sources at once is a substantial addition to this growing list.

A radio galaxy dominates the sky over Earth with radio waves. Radio galaxies emit intense radio waves originating from expansive lobes of gas, extending millions of light-years beyond the visible galaxy structure. Radio galaxies can have luminosities up to 1039 W at radio wavelengths between 10 MHz and 100 GHz. The interaction between the charged particles and strong magnetic fields around the supermassive black holes is what causes the radio emissions. The radio lobes usually occur in pairs and dominate the sky at radio wavelengths. Cygnus A is the first and brightest radio galaxy. In August 2023, a team of astronomers from the National Centre of Radio Astrophysics (NCRA), the Physical Research Laboratory (PRL), and the University of Oxford discovered several "elusive dying radio galaxies.". 

In order to better comprehend the origin and evolution of radio galaxies, four Bengali astrophysicists have been working nonstop for the past four years. Giant radio galaxies are thought to be in the final stages of galaxy evolution, a subject in need of further research. Scientists are better able to comprehend the connection between radio galaxy evolution and black hole activity by examining these huge radio galaxies. This includes the impact that supermassive black holes have on the galaxies they orbit. About 90% of the radio sky was surveyed during the TIFR GMRT Sky Survey (TGSS), which was conducted between 2010 and 2012 using the GMRT to map the radio sky at 150 MHz. The team’s recent discovery was based on the TIFR GMRT Sky Survey (TGSS) Alternative Data Release 1. 

Sushanta Kumar Mondal, assistant professor of physics at Sidho-Kanho-Birsha University, Purulia, Sauvik Manik and Nitai Bhukta, both PhD researchers, and Professor Sabyasachi Pal, Head of the Department of Pure and Applied Science at Midnapore City College, led the four astrophysicists in their discovery of 34 massive radio galaxies using local technology.

For the experiment, the scientists employed 30 specialist radio telescopes, dubbed the Giant Meter-wave Radio Telescope (GMRT), each with a 45-metre diameter. They were employed at the National Centre for Radio Astrophysics (NCRA), an astronomy research facility in India run by the Tata Institute of Fundamental Research (TIFR), which is located close to Khodad hamlet, roughly 90 km north of Pune.

For four years, the astrophysicists studied and examined the radio waves emitted by these massive galaxies and then announced their findings. The discovery was published in the Astrophysical Journal Supplement Series (ApJS) of the American Astronomical Society (AAS). Detecting such giant radio galaxies is challenging because the bridge connecting the two lobes is often not visible. Incidentally, two of the 34 objects found defy the widely accepted notion that GRSs grow in low-density environments. The researchers emphasised that the environment alone does not play a major role in the exceptionally large size of the GRSs.

There are two distinct types of radio galaxies, defined by differences in their optical emissions. Broad-line radio galaxies show broad-line light emissions from ionised oxygen, hydrogen, and silicon in their optical spectrum. Narrow-line radio galaxies are AGNs that lack broad-line emissions but have narrow emission lines from hydrogen and triple-ionised oxygen.

Giant Radio Sources (GRS) provide a unique opportunity to study the intergalactic medium and the large-scale structure of the universe. Their massive size and powerful radio emissions can reveal the distribution and behaviour of matter in the cosmos, offering clues about the interaction between super-massive black holes and their host galaxies. The discovery of these GRSs aids in understanding the late stages of radio galaxy evolution. As radio sources grow larger, their lobes become more difficult to detect, particularly at higher frequencies. The GMRT’s low-frequency observations allow astronomers to identify these sources, even when the connecting emissions are faint or absent.

The discovery of radio galaxies occurred in the late 1930s and early 1940s, when radar operators during the Second World War accidentally started to pick them up. It would take another 10 years, after radio astronomy burgeoned, to better understand what had been discovered.

The first radio galaxy ever detected was Cygnus A, discovered by one of the pioneers of radio astronomy, Grote Reber, in 1939, a featureless oval elliptical galaxy of the same name located around 500 million light-years away, according to Britannica. It remains one of the closest and brightest radio galaxies ever discovered.

Cygnus A consists of two characteristic lobes emerging from a compact galactic nucleus. On the right, a jet is emerging from this nucleus that is feeding energy to the right lobe. Present but less conscious is a second jet to the left of the nucleus, supplying energy to the left lobe. These jets are emerging from a super-massive black hole at the heart of Cygnus A, which is thought to have a mass equivalent to 2.5 billion suns. Cygnus A is one of the most prominent examples of a radio galaxy, but other exceptional radio-loud super-massive black hole-powered galaxies exist in the universe. 

Centaurus A, also known as Caldwell 77 or NGC 5128, is located around 12 million light-years from Earth, making it one of the closest radio galaxies to us and the fifth brightest galaxy in the sky over Earth. One extraordinary feature of this radio galaxy is the thick lane of dust that runs through it, obscuring its AGN heart. This is thought to be the result of a merger with another galaxy, Centaurus A, about 500 million years ago. 

Then there is M87, also known as Virgo A, a radio galaxy that is also home to one of the most famous black holes in the history of astronomy. The super-massive black hole in M87, which has a mass around 4,5 billion times that of the sun, isn't just launching jets that power the radio emissions of this galaxy; it is also the first black hole ever imaged in humanity, seen by the Event Horizon Telescope in 2017 and released to the public in 2019. The radio lobes of M87 billow out for an estimated 130,000 light years.

The team plans to present new GRS samples in forthcoming articles with detailed physical properties based on multi-wavelength observations to unveil further mysteries. This kind of study is ideal for gaining a better understanding of radio galaxy evolution, galactic dynamics, and the intergalactic medium.