PUNE: Scientists at the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research (NCRA-TIFR, Pune) and the Indian Institute of Science Education and Research, Mohali (IISER- Mohali) have used the upgraded Giant Metrewave Radio Telescope GMRT ) to carry out the most distant measurement of the atomic hydrogen content of galaxies in the early Universe , informed NCRA.The GMRT is built and operated by NCRA-TIFR and is among the largest radio telescopes in the world. Galaxies like the Milky-way are made up of stars and gas. The life cycle of a galaxy is essentially the conversion of gas to stars through gravitational collapse of gas clouds.Understanding how galaxies form and evolve thus requires measurements of how both their stars and gas evolve with time. Over the last two decades, astronomers have accurately measured the stellar masses and star formation rates of populations of galaxies at different epochs in the Universe. However, little is known about the evolution of the atomic gas content of galaxies, the “fuel” for star formation.Scientists at the NCRA-TIFR and IISER-Mohali have used the upgraded GMRT to measure gas contents in most distant galaxies. Apurba Bera, a Ph.D. student at NCRA-TIFR and the lead author of the paper, said, “Most of the atomic gas in galaxies is in the form of hydrogen, which emits a characteristic spectral line at the radio wavelength of 21.11 cm. Unfortunately, this hydrogen emission is weak, and even powerful radio telescopes like GMRT do not have sufficient sensitivity to detect the emission from very distant galaxies. However, the wide frequency coverage of the upgraded GMRT allowed to add the hydrogen emission signals from a large number of galaxies in the field of view of the telescope, so as to measure their average gas content.”.The study resulted in a measurement of the average atomic gas content of star-forming galaxies located 4 billion light years away, when the Universe was about two-thirds of its current age. Remarkably, the authors found that both the star-formation efficiency of galaxies and the cosmological gas mass density in galaxies appear to have not changed significantly over the last 4 billion years.Jayaram Chengalur, a professor at NCRA-TIFR and a co-author of the paper, said “The measurement critically requires simultaneous observations of a large number of galaxies. This was possible due to the recent upgrade of the GMRT, as its large bandwidth and superb digital systems allowed us to cover more than 400 galaxies simultaneously.” Jasjeet Singh Bagla, a professor at IISER-Mohali and also a co-author, mentioned that his interest in such studies was initiated by simulations of structure formation in the Universe, and that it was wonderful to see that direct measurements of gas masses of distant galaxies are possible today."The results have been published in the Astrophysical Journal Letters of the American Astronomical Society (AAS), and have been highlighted by AAS Nova as one of the most interesting recent results to appear in journals published by them.