Discovery of Relic Lobes using GMRT

February 2013

A new, rare, fossil structure is seen in the radio image of the famous radio galaxy, called 3C452, in the data taken with the Giant Metrewave Radio Telescope (GMRT), near Pune. This discovery demonstrates that this galaxy had an earlier phase of activity which had produced giant blobs of radio emitting plasma extending over 3 million light-years. This discovery demonstrated that the recurrent activity to produce radio emission in galaxies may be more common than previously thought and has major implications for the impact radio galaxies had on star and galaxy formation in the universe.

The most massive galaxies in the universe have an elliptical appearance devoid of a stellar disk or spiral arms, unlike our own galaxy. A few percent of the elliptical galaxies are seen to become `radio galaxies' by entering a radio-loud phase which lasts about 10 million years. During this phase their radio-frequency emission shoots up by thousands of times or more, and even exceeds its power in the optical band. This is because the supermassive black hole at the center of the galaxy becomes active, ejecting oppositely directed jets of relativistic plasma and magnetic field. These jets can grow out to distances of millions of light-years by boring their way through the gaseous medium between galaxies and filling it with their relativistic plasma that radiates radio waves via the synchrotron mechanism. Thus, a powerful radio galaxy displays two lobes of radio emission located on opposite sides of the active elliptical galaxy.

The best evidence for repetitive nature of the radio-loud phase of galaxies comes from the discovery of `double-double' radio galaxies (DDRGs) in which not one but two pairs of radio lobes are seen. Since only a handful of DDRGs have been found so far, the key question is whether such giant `extra' pairs of lobes exist in many radio galaxies but have remained undetected because of their old age leading to faintness. Since the standard radio galaxies are far more numerous when the universe was a fraction of its present age, it is important to check whether the outer `fossil' radio lobes are a common occurrence in them. This has important implications for their role in the evolution of the universe because the cumulative energy input from such radio jets and lobes can play a vital role in the formation of galaxies and stars in the universe. It is thus crucial to find out just how much external gas is pushed aside by the jets of a typical radio galaxy and for how long.

In this context, the recent discovery of giant fossil radio lobes in a well known classical double radio galaxy, called 3C452, is specially relevant. Over the past 3 decades, this object located at a distance of one billion light-years, has been imaged with many large radio telescopes. Since all these radio images have shown just one pair of radio lobes, which is the expected norm, this object has all along been accepted as text-book example of standard (classical) radio galaxies. However, a refined analysis of the observations taken at a low radio-frequency of 325 Megahertz, using the Giant Metrewave Radio Telescope (GMRT), has now revealed a giant outer pair of radio lobes. These two peripheral radio lobes are clearly the 'relic' from a previous episode of jet activity in this galaxy, as indicated by their `aged' radio emission which is, much stronger at the low radio frequencies where GMRT operates. The example of 3C452 revealed by GMRT may turn out to be a forerunner of similar discoveries expected from upcoming radio telescopes elsewhere in the world.

Located at Khodad near Pune (India), GMRT was built and is operated as an international research facility by the National Centre for Radio Astrophysics (NCRA), Pune of the Tata Institute of Fundamental Research. Currently, GMRT is the world's largest radio telescope operating at metre wavelengths.

This research is to be published in the March 1, 2013 issue of Astrophysical Journal Letters, and is on-line since February 13. It is a result of collaborations between scientists, Dr. S. K. Sirothia and Dr. Gopal Krishna of NCRA.TIFR (Pune, India) and Dr Paul J. Wiita (The College of New Jersey, USA).

Attachment: A color photograph of the 325 MHz GMRT image of 3C452.

Title: The surprising discovery of giant fossil radio lobes around a well known `classical' radio galaxy The outer patch of emission (green color) is the 'relic' emission, due to aged population of electrons, from the earlier phase of activity.



Dr. S. K. Sirothia - 020-2571 9213/9313

Prof. Gopal-Krishna - 020-2571-9240

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