The radio image shows the central region of the cluster of galaxies named “Abell2065” observed with the band-3 (250-500 MHz) of the uGMRT. (HT)
The radio image shows the central region of the cluster of galaxies named “Abell2065” observed with the band-3 (250-500 MHz) of the uGMRT. (HT)

NCRA-TIFR scientist discovers remnant radio galaxy

PUNE: A scientist at the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research (NCRA-TIFR), Pune - Dr Dharam Vir Lal – has discovered a remnant radio galaxy in the peripheral region of a cluster of galaxies named “Abell2065” using the upgraded Giant Metrewave Radio Telescope (uGMRT) and the Chandra X-ray Observatory
By Dheeraj Bengrut
UPDATED ON JUL 30, 2021 05:09 PM IST

PUNE: A scientist at the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research (NCRA-TIFR), Pune - Dr Dharam Vir Lal – has discovered a remnant radio galaxy in the peripheral region of a cluster of galaxies named “Abell2065” using the upgraded Giant Metrewave Radio Telescope (uGMRT) and the Chandra X-ray Observatory. The research has been recently published in the July 16, 2021 issue of the Astrophysical Journal.

About the discovery, Dr Dharam Vir Lal said, “The remnant phase of a radio galaxy is short and only a few such remnants are known in the sky. This discovery showcases the capability of the uGMRT to discover more such objects. Finding more remnant and restarted radio galaxies is important for shedding light on their dynamics and evolution and by extension, the duty cycle of active galaxies. We know that active galaxies are rare and that remnant radio galaxies are rarer because they are short-lived. As a result of their rarity, this dying phase remains a poorly understood phenomenon. By increasing the detection of these rare sources, we will be able to understand different evolution histories of active galaxies and different mechanisms that drive the evolution of the remnant namely, the dying phase of the active galaxy.”

“Combining the images in the radio band using the uGMRT and in the X-ray band using the Chandra X-Ray Observatory, a hint of possible shock is seen across the remnant radio galaxy. It is likely that the radio emission is re-energised by the passage of possible shock front, and that shows the expected change in radio emission characteristics of the newly-discovered remnant radio galaxy,” Dr Dharam Vir Lal said.

For the unversed, a galaxy is a system of stars, gas, and dust bound together by mutual gravity. These galaxies come in different shapes, but the basic structure is the same - a dense core of stars called the nucleus surrounded by stars and gas. However in a small fraction of galaxies, the core is bright and shining with power equivalent to trillions of suns, easily outshining the rest of the light of the combined galaxy. A galaxy that emits such tremendous amounts of energy is called an active galaxy.

It is believed that at the centre of such active galaxies is a supermassive black hole having a mass that is million to billion times the mass of (our) Sun. As this supermassive black hole attracts more material from its neighbourhood, it also flashes away long jets of matter and energy from the core up to a distance of millions of light years. These special, active galaxies exhibit strong emission in the radio wavelengths with sizes much larger than seen in the optical wavelength. The long powerful jets responsible for radio emission can also have a substantial energetic impact on both their host galaxy and the surrounding medium beyond the host galaxy.

The active phase of such galaxies can last for several tens of million years, after which the nuclear activity stops, and the radio emission starts to fade away. This phase of a radio galaxy represents the final dying phase of the active galaxy, often termed as the remnant or dying phase, and begins once nuclear activity switches-off. This dying phase is relatively short-lived and is difficult to detect. Fortunately, it remains observable for many tens of million years at low radio frequencies, using a sensitive low-frequency radio telescope such as the uGMRT.

The GMRT is an array of thirty 45m antennas spread over a 25 square kilometre area in Khodad village, Narayangaon, India, built and operated by the NCRA-TIFR, Pune. Currently, it is one of the most sensitive low-frequency radio telescopes in the world.

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