Bengaluru scientists peek into 'Cosmic Dawn'

Scientists observed radiation from hydrogen atoms in and around the galaxies, emitted at a frequency of approximately 1420 MHz

Astronomers from Bengaluru together with scientists from the UK and Israel have peeked into the ‘Cosmic Dawn’ and discovered the properties of the very first set of galaxies formed 200 million years ago after the Big Bang.

Their study – published in Nature Astronomy on Monday – provides answers to key queries on the mass and brightness of early galaxies besides telling the world what fraction of hydrogen gas available in the baby universe was converted into stars.

Saurabh Singh and Ravi Subrahmanyan from Raman Research Institute (RRI), Bengaluru along with collaborators at the University of Cambridge and the University of Tel-Aviv, used data from a portable home-grown radio telescope (SARAS 3) to throw light on the energy output, luminosity, and masses of the first generation of galaxies that are bright in radio wavelengths.

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“Our study shows not more than 3 per cent of hydrogen gas gets converted into stars in the early universe,” Singh told DH. “Also the masses of the first-generation galaxies couldn’t go beyond 300 million times the solar mass. And they can’t be more than 1000 times brighter than the current brightness of the Milky Way and other galaxies like Andromeda and Magellanic Cloud.”

Scientists study the properties of very early galaxies by observing radiation from hydrogen atoms in and around the galaxies, emitted at a frequency of approximately 1420 MHz - also known as the 21 cm line in astronomy parlance.

The radiation is stretched by the expansion of the universe, as it travels across space and time, and arrives at Earth in lower frequency radio bands of 50-200 MHz, also used by FM and TV transmissions.

Since the cosmic signal is extremely faint, buried in orders of magnitude brighter radiation from our own galaxy and man-made terrestrial interference, detecting the signal, even using the most powerful existing radio telescopes, has remained a challenge for astronomers.

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The RRI team designed a unique telescope known as SARAS-3 and deployed it at Dandiganahalli Lake and Sharavathi backwaters in early 2020. They decided to float the telescope to reduce signal contamination and collected nearly 100 hours of data. But even then they could use only 15 hours of data as a lot of signals were heavily contaminated.

"The results from the SARAS 3 telescope are the first time that radio observations of the averaged 21-centimeter line have been able to provide an insight to the properties of the earliest radio-loud galaxies that are usually powered by supermassive black holes," said Subrahmanyan, former director of RRI, who is currently with Space and Astronomy, CSIRO, Australia, and one of the authors of the paper.

Singh said SARAS 3 improved human understanding of astrophysics of Cosmic Dawn when stars formed for the first time around 250-350 million years after the Big Bang. “It tells us that less than 3 per cent of the gaseous matter within early galaxies was converted into stars and that the earliest galaxies that were bright in radio emission were also strong in X-rays, which heated the cosmic gas in and around the early galaxies.”

The scientists have further refined the telescope and are now looking for a site preferably in the hills with minimum background FM signatures for the next round of observations.