Radio signal emitted by atomic hydrogen in distant galaxy detected

Atomic hydrogen is the basic fuel required for star formation in a galaxy.

The illustration shows detection of the lensed 21 cm atomic hydrogen emission signal from a distant galaxy. | Photo Credit: Swadha Pardesi

USING data from the Giant Metrewave Radio Telescope (GMRT) near Pune, astronomers from the Indian Institute of Science (IISc), Bengaluru, and McGill University, Canada, have detected a radio signal emitted by atomic hydrogen in an extremely distant galaxy. The signal has traversed an astronomical distance that is the largest so far by a fair margin. The findings were published in Monthly Notices of the Royal Astronomical Society.

Atomic hydrogen is the basic fuel required for star formation in a galaxy. When hot ionised gas from the interstellar medium around a galaxy falls onto the galaxy, the gas cools and forms atomic hydrogen, which then becomes molecular hydrogen and eventually leads to the formation of stars. To understand the evolution of galaxies over cosmic time, it is necessary to trace the evolution of neutral gas at different cosmological time scales.

Atomic hydrogen emits radio waves with a wavelength of 21 cm, which can be detected using low-frequency radio telescopes like the GMRT. Thus, a 21 cm emission is a direct tracer of the atomic gas content in both nearby and distant galaxies. However, this radio signal is extremely weak and difficult to detect it from a distant galaxy using current telescopes. Until now, the most distant galaxy detected using 21 cm emission was at redshift z = 0.376. This corresponds to a look-back time—the time elapsed between detecting the signal and its original emission—of 4.1 billion years. Redshift represents the change in wavelength of the signal depending on the object’s location and movement: greater the value of z, the farther the object.

Using the GMRT data, Arnab Chakraborty of McGill University and Nirupam Roy of the IIScdetected the radio signal with redshift z = 1.29. The detected signal corresponds to a look-back time for the source of 8.8 billion years. By the time the signal travelled from the source to the telescope, the 21 cm emission line had redshifted to 48 cm.

This detection was made possible by a phenomenon called gravitational lensing, in which the light emitted by a source gets bent owing to the gravitational force of another intervening massive body between the target galaxy and the observer. This results in the “magnification” of the signal. “In this specific case, the magnification of the signal was about a factor of 30, allowing us to see through the high redshift universe,” explains Roy. This is the first confirmed detection of “strong lensing” of 21 cm emission from a galaxy.