Why are fewer stars forming now? Indian astronomers have the answer

In a landmark discovery, Pune and Bangalore astronomers have used India's biggest radio telescope to pick up extremely faint signatures of elusive hydrogen gas from an epoch 8 billion years ago to answer a key question - why fewer stars are forming now than in the early universe?

Using the Giant Metrewave Radio Telescope near Pune, the scientists studied 7,653 galaxies and found that a few billion years after the Big Bang, those massive star clusters contained about 2.5 times more hydrogen than galaxies of today.

Such an excess of hydrogen was the reason for the high rate of star formation in the early universe.

The star formation activity in galaxies peaked about 8-10 billion years ago and has been declining steadily since then. The cause of this decline is unknown, mostly because there is hardly any information about the amount of atomic hydrogen gas, the primary fuel for star formation, in galaxies in these early times.

What scientists at the National Centre for Radio Astronomy and Raman Research Institute did is to measure for the first time, the atomic hydrogen gas content of star-forming galaxies about 8 billion years ago, using the upgraded GMRT.

“To understand the evolution of the galaxies, we need to know about the stars and gases. While we do have good ideas about stars, we know barely anything about the gas (atomic hydrogen) as they are hard to detect. The measurement of atomic hydrogen will now provide a complete picture on the evolution of the galaxies,” Jayaram Chengalur, NCRA scientist and a team member told DH.

“Given the intense star formation in these early galaxies, their atomic gas would be consumed 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 Aditya Chowdhury, a PhD student at NCRA and lead author of the study. “The observed decline in star formation activity can thus be explained by the exhaustion of the atomic hydrogen.”

This explains why far more star formation happened 8-10 billion years ago than now. In fact, more than half of the stars seen in the night were of that vintage, said Nissim Kanekar, another team member from NCRA.

The discovery was made after the radio telescope was upgraded to improve its sensitivity and outreach.

GMRT was set up in the 1990s by the pioneering Indian radio astronomer Govind Swarup who passed away last month and discovery of the signature of atomic hydrogen, incidentally, was one of GMRT’s scientific goals.

But the radio telescope in its original avatar was not sensitive enough to catch such a feeble signal coming from the other end of the universe.

Indian astronomers used a new method to capture the signals of the ancient hydrogen gas because of the upgraded GMRT’s ability to look at nearly 8000 galaxies for a far longer time. In its original form it could see only around 800 galaxies in a similar sky-scan.

“It is a watershed moment in our understanding of how regular (baryonic) matter is taken up and used by the galaxies,” commented Chris Carilli from the National Radio Astronomy Observatory, New Mexico, USA who is not associated with the Indian research work.

The study has been published in the journal Nature on Wednesday.