Recently a team of astronomers from India have reported discovery of a large number of extremely rare kind of galaxies called “giant radio galaxies” (GRGs), using a nearly 20 year old radio survey. GRGs are the largest galaxies known in the Universe, which are visible only to radio telescopes. These extremely active form of galaxies harbor a super massive black hole ‘central-engine’ at the nucleus, which ejects a pair of high energy particle jets nearly at the speed of light, which terminate into two giant radio lobes. These behemoths span nearly three million light years across, or even more sometimes. This size corresponds to stacking nearly 33 Milky Way like galaxies in a line!
Pratik Dabhade said, “The huge size of GRGs has defied any theoretical explanation so far. Our work will help in understanding how these galaxies grow to be so large. We are studying whether they are born in regions of very sparse galaxy density, or they have extremely powerful, well-collimated, long-lasting radio jets which allow them to expand to very huge distances.
Professor Joydeep Bagchi added, “understanding the life-cycle of the black hole’s energetic activity, properties of the matter which falls into it, and the influence of the surrounding medium which acts on the lobes far away from the host galaxy, and provides a ‘working-surface’ for the radio jets to act, are among the most important problems in this field.”
Since the GRGs are known to expand to such large sizes, they are believed to be the last stop of radio galaxy evolution. Astronomers have found many thousands of smaller radio galaxies in the past six decades, but only a handful of GRGs are known so far. The first GRG was discovered in the 1970s using the Westerbork Synthesis Radio Telescope in the Netherlands in 1974. Since then all major radio telescopes and powerful computer simulations have been used in an effort to unravel their mysterious nature.
In the published work, astronomers used the data from a highly sensitive radio survey at 1.4 Gigahertz frequency, done with the Very Large Array (VLA) telescope in New Mexico, USA. Hundreds of new GRGs were discovered by the team. Most of these newly found GRGs are highly unusual, showing very powerful radio jets, feeding large, diffuse radio lobes. Their discovery involved careful inspection of vast amounts of radio data but also sensitive optical spectroscopy and imaging data to identify the host galaxy and measure its distance from earth. The team also show that their new GRGs all host super massive black holes with masses of 100 million to billions of solar masses, in which matter is in-spiralling and liberating extraordinary powerful radio wave energy. These exotic giant radio galaxies are now being followed up with Giant Metrewave Radio Telescope (GMRT) near Pune, and using The Low-Frequency Array (or LOFAR), located mainly in the Netherlands and Europe, for more detailed studies.
Featured image credit: Artist’s impression of a supermassive black hole at the centre of a galaxy. CC BY 4.0 via ESO/L. Calçada.