In an important breakthrough, the Nationwide Centre for Radio Astrophysics (NCRA-TIFR) based in Pune and the Raman Research Institute (RRI) in Bengaluru have utilized the Giant Metrewave Radio Telescope (GMRT) to gauge the atomic hydrogen content of galaxies from approximately 8 billion years ago. This discovery could provide key insights into the reasons behind the current decline in star formation within the Milky Way.
Details of the Study
The study marked the first occasion where the atomic hydrogen gas content of galaxies was measured for a date approximately 8 billion years in the past. This feat was achieved with assistance from the upgraded GMRT. Unlike stars, which emit light intensely at optical wavelengths, atomic hydrogen signals are found in radio wavelengths, specifically at a wavelength of 21 cm. These signals can only be detected using radio telescopes.
Introducing the Stacking Technique
The 21 cm signal is inherently very faint. To overcome this challenge, the researchers adopted a method called “stacking”. This technique involves combining the 21 cm signals from nearly 8,000 galaxies that had previously been identified using optical telescopes. Via this methodology, the average atomic gas content of these galaxies was successfully measured.
Significance of the Discovery
Galaxies consist primarily of gas and stars, with gas being converted into stars over the course of a galaxy’s lifespan. To fully comprehend galaxies, it’s crucial to study how the quantities of both gas and stars evolve over time. Star formation activity in galaxies was at its peak around 8-10 billion years ago; however, it has been on the decline ever since.
The cause of this decline remained a mystery due to the lack of information about the amount of atomic hydrogen gas — the main fuel for star formation — present in early galaxies. The newly observed decline in star formation activity could potentially be attributed to the depletion of atomic hydrogen gas. Given the intense star formation in these early galaxies, their atomic gas reserves would likely have been depleted within one or two billion years.
Giant Metrewave Radio Telescope (GMRT)
The GMRT consists of thirty fully steerable parabolic radio telescopes, each with a 45-meter diameter. Operated by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research, it’s an entirely indigenous project. The GMRT operates in the meter wavelength part of the radio spectrum, as man-made radio interference is significantly reduced in this spectrum in India.
Detecting the 21 cm signal from the universe’s most distant galaxies was the primary scientific goal of the GMRT during its design and construction phases in the 1980s and 1990s, under the guidance of late pioneering astrophysicist Govind Swarup. A significant increase in sensitivity was achieved following the upgrade of the GMRT with new wide-band receivers and electronics in 2017.
Pune, the location of the GMRT, was chosen based on several crucial factors including low man-made radio noise, good communication facilities, close proximity to industrial and educational infrastructure, and a geographical latitude sufficiently north of the geomagnetic equator. This ensures a reasonably stable ionosphere and optimal observation conditions for a significant part of the southern sky.
