India's SARAS Telescope Makes Discoveries About Universe's First Stars And Black Hole-Powered Galaxies

An international team of scientists, including Indian researchers, has determined properties of radio luminous galaxies formed 200 million years after the Big Bang, during the Cosmic Dawn.

India's SARAS (Radio Spectrum 3) telescope has made interesting discoveries about the universe's earliest stars and galaxies. Using the observations of this radio telescope, an international team of scientists has determined properties of radio luminous galaxies formed 200 million years after the Big Bang, during the Cosmic Dawn. Radio luminous or radio-loud galaxies are the active galaxies (galaxies with a small core of emission embedded at the centre) which emit more light at radio wavelengths than at visible wavelengths, and are driven by non-thermal emission. 

What is Cosmic Dawn?

The era 50 million to one billion years after the Big Bang, in which electrons were separated from protons due to gas becoming ionised by the ultraviolet radiation of stars, is known as the Cosmic Dawn. This was the period when the first stars, galaxies and black holes of the universe formed. 

Therefore, the findings of the SARAS telescope provide valuable insights into the properties of the earliest radio-loud galaxies usually powered by supermassive black holes. 

The SARAS telescope was indigenously designed and built at the Raman Research Institute, and deployed in 2020 over Dandiganahalli Lake and Sharavathi backwaters located in Northern Karnataka. 

According to a statement released by the Union Ministry of Science and Technology, a team of researchers from the Raman Research Institute (RRI), Bengaluru and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia, and collaborators at the University of Cambridge and University of Tel-Aviv estimated the luminosity, energy output and masses of the first generation of galaxies that are bright in radio wavelengths. The first-of-its-kind study was published in the journal Nature Astronomy on November 28, 2022.

What does radiation from galaxies tell astronomers?

One can determine the properties of very early galaxies by observing radiation from hydrogen atoms in and around the galaxy, emitted at a frequency of approximately 1,420 megahertz. Due to the expansion of the universe, the radiation is stretched as it travels to Earth across space and time. By the time the radiation reaches Earth, its frequency decreases to 50 to 200 megahertz. This is the frequency range used by FM (frequency modulation) and TV transmissions. 

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

SARAS has provided insights into radio luminous galaxies

In the statement, Ravi Subrahmanyan, former director of RRI and currently with CSIRO, Australia, and an author on the paper, said the results from SARAS 3 mark the first time that radio observations of the averaged 21-centimetre line have been able to provide an insight into the properties of the earliest radio-loud galaxies that are usually powered by supermassive black holes. He added that the new work takes forward the results from SARAS 2, which was the first to inform the properties of earliest stars and galaxies. The 21-centimetre line is a cosmic signal from some of the very first galaxies. 

Saurabh Singh from RRI, and one of the authors on the paper, said SARAS 3 has improved researchers' understanding of the astrophysics of the Cosmic Dawn. The findings tell researchers that less than three per cent of the gaseous matter within early galaxies was converted into stars. 

He added that the earliest galaxies that were bright in radio emissions were also strong in X-rays. The X-rays heated the cosmic gas in and around the early galaxies. 

In March this year, the researchers used the same data to reject claims of the detection of an anomalous 21-centimetre signal from Cosmic Dawn made by the EDGES radio telescope developed by researchers from Arizona State University (ASU) and Massachusetts Institute of Technology, United States.

Dr Anastasia Fialkov from the Institute of Astronomy, University of Cambridge, and an author on the paper, said the analysis has shown that the 21-centimetre hydrogen signal can inform about the population of first stars and galaxies, and places limits on the some of the key properties of the first sources of light, including the masses of the earliest galaxies and the efficiency with which these galaxies can form stars.