Upgraded GMRT gives new insights into galaxy evolution

Astronomers from India’s National Centre for Radio Astrophysics (NCRA-TIFR) in Pune, and the Raman Research Institute (RRI) in Bangalore, have used the upgraded Giant Metrewave Radio Telescope (GMRT), an SKA pathfinder facility, to measure for the first time the atomic hydrogen content of galaxies seen as they were eight billion years ago.

This is the earliest epoch in the Universe for which there is such a measurement, giving new insights into how galaxies evolve. The research has been published in the journal Nature.

Atomic hydrogen gas is the primary fuel for star formation. Astronomers have long known that galaxies formed stars at a higher rate when the Universe was young than they do today, peaking about 8-10 billion years ago and declining steadily ever since. The cause of this decline is unknown, mostly because – until now – there was no information about the amount of atomic hydrogen in galaxies in these early times.This new measurement has helped to fill that crucial gap.

“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,” says Aditya Chowdhury, a PhD student at NCRA-TIFR and the study’s lead author. “The observed decline in star formation activity can thus be explained by the exhaustion of the atomic hydrogen.”

Unlike stars which emit light strongly at optical wavelengths, the atomic hydrogen signal, with a wavelength of 21cm, can only be detected with radio telescopes. However, the signal is extremely weak and difficult to detect from distant individual galaxies even with powerful telescopes like the upgraded GMRT. To overcome this, the team used a technique called “stacking” to combine the 21cm signals of nearly 8,000 galaxies that had earlier been identified with optical telescopes. This method measures the average gas content of these galaxies.

This study was made possible by GMRT’s new wideband receivers and electronics, which provided a boost in sensitivity enabling the telescope to use 10 times more galaxies in the stacking analysis than a previous similar study.

This is the kind of science that the upgraded GMRT promised, and I am really pleased that we are now beginning to deliver. This kind of science is also one of the main drivers of the SKA. So, true to its role as a SKA pathfinder facility, the upgraded GMRT is making discoveries that will prepare us for even more exciting science with the SKA!

Prof. Yashwant Gupta, Director of NCRA

By NCRA

An image of the stacked 21cm signal detected with the upgraded GMRT, arising from atomic hydrogen gas in galaxies 22 billion light years away.

Credit: NCRA

The spectrum of the stacked 21cm signal detected with the upgraded GMRT, arising from atomic hydrogen gas in galaxies 22 billion light years away. The width of the signal gives an indication of the average rotation of galaxies eight billion years ago.

Credit: NCRA