What’s in the news?

• In a vital discovery which may help understand the mystery behind declining star formation activity in the Milky Way, a team of astronomers from the Pune-based National Centre for Radio Astrophysics (NCRA-TIFR) and Raman Research Institute (RRI) in Bengaluru have used the upgraded Giant Metrewave Radio Telescope (GMRT) to measure the atomic hydrogen content of galaxies.

Importance of their study

• Galaxies in the universe are made up mostly of gas and stars, with gas being converted into stars during the life of a galaxy.
• Understanding galaxies requires us to determine how the amounts of both gas and stars change with time.
• Astronomers have long known that galaxies formed stars at a higher rate when the universe was young than they do today. The star formation activity in galaxies peaked about 8-10 billion years ago and has been declining steadily till today.
• The cause of this decline was unknown as there had been no information regarding the amount of atomic hydrogen gas — the primary fuel for star formation — in galaxies in these early times.
• For the first time, Indian astronomers measured the atomic hydrogen gas content of star forming galaxies about 8 billion years ago, using the upgraded GMRT. Given the intense star formation in these early galaxies, their atomic gas would be consumed by star formation in just one or two billion years. And, if the galaxies could not acquire more gas, their star formation activity would decline, and finally cease.
• Unlike stars which emit light strongly at optical wavelengths, the atomic hydrogen signal lies in the radio wavelengths, at a wavelength of 21 cm, and can only be detected with radio telescopes.
• Unfortunately, this 21 cm signal is intrinsically very weak, and difficult to detect from distant individual galaxies even with powerful telescopes like the upgraded GMRT. To overcome this limitation, the team used a technique called “stacking” to combine the 21 cm signals of nearly 8,000 galaxies that had earlier been identified with the help of optical telescopes. This method measures the average gas content of these galaxies.

Additional Details

• Located near Pune, the GMRT is a radio telescope. As with most powerful telescopes, it comprises a group of dishes or an “array” that are all steerable and can turn their giant heads in any direction. A radio telescope is a specialized antenna and radio receiver used to receive radio waves from astronomical radio sources in the sky
• When all of GMRT’s 30 parabolic dishes point to the same source, the signal that’s received from the source is greatly amplified. This type of operation, where multiple antennae operate as one, is called interferometry.
• In August 2018, the most distant galaxy ever known, located at a distance of 12 billion light years, was discovered by GMRT.
• In February 2020, it helped in the observation of the biggest explosion in the history of the universe, the Ophiuchus Supercluster explosion.