Context: In a vital discovery by National Centre for Radio Astrophysics (NCRA-TIFR) and Raman Research Institute (RRI) may help understand the mystery behind declining star formation activity in the Milky Way.
About the discovery

• The cause of this decline was unknown as there had been no information regarding the amount of atomic hydrogen gas (primary fuel for star formation) in galaxies in these early times.
• For the first time, the atomic hydrogen gas content of star forming galaxies about 8 billion years ago is measured.
• The upgraded Giant Metrewave Radio Telescope (GMRT) has been used for this discovery.
• 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.
• The observed decline in star formation activity could thus be explained by the exhaustion of the atomic hydrogen.
• Unlike stars which emit light strongly at optical wavelengths, the atomic hydrogen signal lies in the radio wavelengths at 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, a technique called “stacking” is usedto combine the 21 cm signals of nearly 8000 galaxies that had earlier been identified with the help of optical telescopes. This method measures the average gas content of these galaxies.

Significance

• For the first time, atomic hydrogen gas content of star-forming galaxies about 8 billion years ago has been measured.
• 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.
• The discovery may help understand the mystery behind declining star formation activity in the Milky Way.

Giant Metrewave Radio Telescope (GMRT)

• GMRT is an array of thirty fully steerable parabolic radio telescopes of 45-metre diameter. It is operated by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research.
• It was designed and built by a team led by the late pioneering astrophysicist Govind Swarup in the 1980s and 1990s.
• Detecting the 21 cm signal from the most distant galaxies in the universe was the main science goal of the GMRT.
• Its design is based on the `SMART’ concept – for Stretch Mesh Attached to Rope Trusses.
• It is erected at Narayangaon, near Pune.
• The location for GMRT, Pune meets several important criteria such as low man-made radio noise, availability of good communication, vicinity of industrial, educational and other infrastructure and, a geographical latitude sufficiently north of the geomagnetic equator in order to have a reasonably quiet ionosphere and yet be able to observe a good part of the southern sky as well.