A team of
astronomers working at the National Centre for Radio Astrophysics (NCRA,
TIFR), Pune have discovered, using the Giant Metrewave Radio Telescope
(GMRT), an extremely rare galaxy of gigantic size. This galaxy — located
about 9 billion light-years away towards the constellation Cetus —
emits powerful radio waves and has an end-to-end extent of a whopping 4
million light-years.
Such galaxies with extremely large ‘radio size’ are
appropriately called giant radio galaxies.
How do galaxies with an optical size of a hundred thousand
light-years produce radio emission several million light-years in
extent? It is argued that the presence of a supermassive black hole at
the centre of the galaxy drives large scale jets of hot plasma in
diametrically opposite directions which eventually give rise to large
radio lobes (see the image above). While radio galaxies with size less
than a million light-years are common, giant radio galaxies are
extremely rare, even more so, at large cosmic distances where only a
handful have been discovered so far. This newly discovered galaxy known
by its scientific name ‘J021659-044920’ is the newest member of this
elite group.
Under some special circumstances, the central black hole may stop
producing the radio jet, and then the bright radio lobes fade away,
within a few million years, due to lack of replenishment. What makes
J021659-044920 special is that it has been caught in this dying phase,
where the radio jet appears to have switched off and the radio lobes
have started fading. The fading of the lobes is caused by their losing
energy in two ways: one, by emitting radio waves which show up as the
gigantic radio lobes and two, by transferring energy to photons from the
cosmic microwave background via a process known as inverse Compton
scattering. This latter mechanism leads to faint X-ray emission, which
is seen to emanate from the radio lobes of this galaxy. Such dying radio
objects are best studied using a low frequency radio telescope such as
the GMRT. The GMRT, the world’s largest radio telescope facility
operating at low radio frequencies, is an array of 30 fully steerable,
45-metre diameter antennas, spread out over a 30 kilometre region around
Khodad, near Narayangaon town of Pune district in western India. The
GMRT was built and is operated by National Centre for Radio Astrophysics
of the Tata Institute of Fundamental Research and has been in operation
since 2002.
For their analysis, the team combined their GMRT observations with
previous observations made with a slew of international ground and space
based telescope facilities — XMM-Newton Space Telescope in X-ray, the
Japanese Subaru Telescope in optical, UK’s Infrared Telescope in
near-infrared, NASA’s Spitzer Space Telescope in mid-infrared and the
Jansky Very Large Array (USA) in high frequency radio bands. By using
data from multiple telescopes spanning across the electromagnetic
spectrum, they were able to carry out a comprehensive and incredibly
detailed analysis of the physical conditions around this distant galaxy.
The properties of the magnetic field in the region between galaxies in
the distant universe can be understood with these observations.
Source: Astronomy Now
