Pune: Indian astronomers, using the Giant Metrewave Radio Telescope (GMRT) near Pune, recently detected an extremely faint radio signal of atomic hydrogen from galaxies that existed 8-9 billion years ago. The detection is a step towards understanding how stars formed in the early Universe and why star formation has declined over billions of years.Atomic hydrogen is the basic fuel galaxies need to make stars. It combines to form molecular hydrogen, from which stars are formed. Measuring how much atomic hydrogen galaxies contain at different stages of the Universe’s history helps astronomers understand how galaxies grow and why their star formation has slowed. It’s widely believed that star formation has fallen, the peak being 10 billion years ago. It’s estimated that about half of the stellar mass today had already formed by roughly 9 billion years ago.The Indian team that made the recent detection said their findings, published in The Astrophysical Journal Letters, suggest although galaxies in the early Universe had larger reserves of atomic hydrogen, they were also forming stars at a much faster rate than galaxies today. This essentially means they consumed their gas quickly. And without fresh supplies of gas from their surroundings, these galaxies eventually ran short of the material needed to form new stars, members of the team said.The research was carried out by experts from the National Centre for Radio Astrophysics (NCRA-TIFR) and the Inter-University Centre for Astronomy and Astrophysics (IUCAA). They combined signals from nearly 10,000 distant galaxies to measure their average atomic hydrogen content.“We used the GMRT to look at a region in the sky called the COSMOS field for more than 200 hours. We aligned the ‘HI 21 cm’ signals of nearly 10,000 galaxies and then averaged them together to measure the average atomic hydrogen content of the galaxies,” said Balpreet Kaur, lead author of the study and a postdoctoral fellow at IUCAA.The HI 21 cm signal is emitted by atomic hydrogen at a radio wavelength of about 21cm. Radio telescopes can detect this signal and help scientists estimate the amount of hydrogen in galaxies. Since light and radio signals from distant galaxies take billions of years to reach Earth, astronomers observe these galaxies as they existed in the distant past.Kaur said the team’s findings, indicating galaxies run out of star-forming fuel, provide a “natural explanation for the observed drop in the star-formation activity of the universe over the last 8 billion years”.Co-author Aditya Chowdhury said the method helped overcome a major challenge in studying the early universe. “The HI 21 cm signal is so faint that it is extremely challenging to detect it from individual galaxies that existed at these early times. However, the ‘HI 21 cm stacking’ technique combines the signals of many galaxies and allows us to measure their average signal,” said Chowdhury, who completed his PhD at NCRA-TIFR.Co-author Jayaram N Chengalur said the COSMOS field was particularly useful because astronomers already had extensive information about its galaxies from observations made using different types of light. Researchers can now compare their hydrogen content with other properties, including their surroundings, to better understand how galaxies evolve.Nissim Kanekar, another co-author and faculty member at NCRA-TIFR, said the detection was an important step towards measuring the hydrogen signal from individual galaxies from that period — something astronomers have not yet achieved. The team is carrying out deeper observations of the same region using the GMRT in an attempt to reach that goal.
