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 seen as they were eight billion years ago when the universe was young.
The research, carried out by Aditya Chowdhury, noted astrophysicist Nissim Kanekar, and Jayaram Chengalur of NCRA-TIFR, and Shiv Sethi, and K. S. Dwarakanath of RRI, has been published in the October 15 issue of the prestigious British scientific journal Nature .
Explaining the importance behind the research, Mr. Chowdhury, a PhD scholar at NCRA-TIFR and the lead author of the study, said 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,” Mr. Chowdhury said.
Unlocking the mystery
He observed that 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.
“We have, for the first time, 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”, said Mr. Chowdhury, adding that the observed decline in star formation activity could thus be explained by the exhaustion of the atomic hydrogen.
The measurement of the atomic hydrogen mass of distant galaxies was done by using the upgraded GMRT to search for a spectral line in atomic hydrogen.
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.
Commenting on the measurements taken using the upgraded GMRT, Mr. Kanekar said, “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.”
Mr. Kanekar said studying the distant universe through the 21 cm signal has remained an important research area in astronomy, and one of the key science goals of the GMRT.
Mr. Chengalur said the big jump in sensitivity was due to the upgrade of the GMRT in 2017. “The new wide band receivers and electronics allowed us to use 10 times more galaxies [8,000 galaxies were observed in the study] in the stacking analysis, giving sufficient sensitivity to detect the weak average 21 cm signal.”
Detecting the 21 cm signal from the most distant galaxies in the universe was the main science goal of the GMRT, when it was designed and built by a team led by the late pioneering astrophysicist Govind Swarup in the 1980s and 1990s.
