New Remnant Radio Galaxy Discovered at the Peripheral Region of a Cluster of Galaxies

At the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research (NCRA-TIFR), Pune, India, Dr. Dharam Vir Lal, a scientist, has found a remnant radio galaxy at the peripheral region of a cluster of galaxies termed Abell2065.

The central region of the clusters of galaxies named Abell2065 observed with the band-3 (250-500 MHz) of the upgraded GMRT. The radio image is shown in surface-brightness contours overlaid on the visible band image. The two massive galaxies, in the north (+) and the south (+) are shown. The remnant radio galaxy shows an elongated, bar-shaped structure, whose size is approximately a million light-years. The position of the host galaxy is marked (*). Image Credit: Dharam Vir Lal.

Dr Lal achieved this by making use of the upgraded Giant Metrewave Radio Telescope (uGMRT) and the Chandra X-ray Observatory. The study was published in The Astrophysical Journal on July 16th, 2021.

A galaxy is a collection of stars, dust, and gas particles bound by their mutual gravity. Such galaxies can be seen in different shapes, but the fundamental structure appears to be the same—a dense core of stars known as a nucleus encircled by stars and gas.

But in a small fraction of galaxies, the core becomes bright, shining with a power corresponding to trillions of suns, readily outclassing the rest of the light of the galaxy together. A galaxy that has the potential to discharge such vast amounts of energy is known as an active galaxy.

It is thought that at the core of such active galaxies rests a supermassive black hole having a mass of million to billion times the mass of the Sun. This supermassive black hole draws more material from its neighborhood and also streaks away long jets of energy and matter from the core up to a distance of millions of light-years.

These unique and active galaxies display powerful emissions in the radio wavelengths with a size much higher compared to what is observed in the optical wavelength. The long robust jets accountable for radio emission can also exhibit a considerable energetic impact on both their host galaxy and the encircling medium present beyond the host galaxy.

The active phase of these galaxies has the potential to last several tens of millions of years, following which the nuclear activity is hindered and the radio emission begins to vanish. This phase of a radio galaxy constitutes the active galaxy’s final dying phase, generally called the dying or remnant phase. This phase commences as soon as the nuclear activity ceases. This dying phase is much short-lived and hard to detect.

With the help of a sensitive low-frequency radio telescope, like the upgraded GMRT, the galaxies can still be observed for several tens of millions of years at low radio frequencies.

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 upgraded GMRT to discover more such objects.

Dharam Vir Lal, Scientist, National Center for Radio Astrophysics, Tata Institute of Fundamental Research

Discovering more remnant and restarted radio galaxies is essential to provide insights into their dynamics and evolution, and thus the duty cycle of active galaxies.

Researchers are well aware that active galaxies are rare (around 10% of all galaxies) and that remnant radio galaxies are even rarer since they are short-lived. Due to their rarity, this dying phase is still a phenomenon that is understood poorly.

Increasing the detection of such rare sources will help researchers comprehend the various evolution histories of active galaxies and of different mechanisms that power the remnant’s evolution, that is, the dying phase of the active galaxy.

Integrating the images in the radio band captured with the upgraded GMRT and in the X-ray band captured with the Chandra X-ray Observatory, a trace of possible shock is observed across the remnant radio galaxy.

Probably, it is the radio emission that has been re-energized by the transit of a possible shock front, and it also exhibits the anticipated variation in radio emission properties of the newly found remnant radio galaxy.

The GMRT includes a series of thirty 45-m antennas distributed over a 25-km2 area located in Khodad village, Narayangaon, India. It is constructed and run by NCRA-TIFR, Pune. At present, it is considered to be one of the most sensitive low-frequency radio telescopes in the world.

Journal Reference:

Lal, D. V., (2021) The Discovery of a Remnant Radio Galaxy in A2065 Using GMRT. The Astrophysical Journal.

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