E-Paper
UVIT onboard Astro Sat captures escaping ionising photons from galaxies
Detecting ionising UV radiation from such galaxies is extremely challenging, and was possible only because of the unique capabilities and high sensitivity of the UVIT, says author
An international team of astronomers has been successful in detecting ionising photons from rare type of galaxies known as ‘Lyman Continuum Leakers’ using the Ultraviolet Imaging Telescope (UVIT) onboard the Astro Sat. “Detecting ionising UV radiation from such galaxies is extremely challenging, and was possible only because of the unique capabilities and high sensitivity of the UVIT,” said Suraj Dhiwar, lead author of the research published in ‘The Astrophysical Journal Letters’.
The team of researchers includes Dhiwar, a research scholar at the Inter-University Centre for Astronomy and Astrophysics (IUCAA) Pune and Ph.D. student at the Savitribai Phule Pune University; professor Kanak Saha, Soumil Maulick and Dr Chayan Mondal from the IUCAA Pune; Dr Brent Smith and professor Rogier Windhorst from Arizona State University, USA; professor Marc Rafelski of NASA’s Space Telescope Institute, USA; and professor Harry Teplitz from Caltech, USA.
In the first billion years after the Big Bang, our Universe went through a major transition known as reionisation, a process in which neutral hydrogen atoms dissociated into protons and electrons when they were struck by high energy UV radiation below wavelengths 912 Å known as Lyman Continuum emission. Understanding the cosmic reionisation and sources responsible for this process remains a major challenge in astronomy.
“The Lyman continuum emission can be easily absorbed or scattered by the interstellar medium or the circumgalactic medium of their host galaxies. Even when some of these ionising photons manage to come out of the galaxy’s environment, they may be absorbed by the vast intergalactic medium between us and the galaxy. This is what makes their discovery a rare event in astrophysics. Thanks to the UVIT’s resolution and sensitivity that allowed us to create UV deep field in the far ultraviolet filter,” said professor Kanak Saha.
In the current discovery, astronomers detected 10 Lyman continuum emitting galaxies from the peak era of cosmic star formation history, making it the first coherent sample of Lyman continuum leakers at this epoch. More interestingly, these Lyman Continuum photons have a wavelength ~600 Å, falling in the extreme ultraviolet regime, the shortest ultraviolet wavelength with which a galaxy has been imaged so far. These galaxies are about eight to nine billion light years away from the Earth and have intense star formation rates, with some of them forming massive young stars at a rate 100 times higher than our Milky Way Galaxy.
Prof Rogier Windhorst said, “The discovery would fill an important niche in understanding the evolution of these rare objects, which are at an epoch when the star formation was at its peak in the cosmic star formation history.”
Beside the UV observation from Astro Sat, the Hubble Space Telescope was used to obtain the optical/infrared imaging and spectroscopy for these rare galaxies. The research utilised the deep ultraviolet observations made by professor Kanak Saha in 2018 using the UVIT onboard the Astro Sat, and was funded by the Indian Space Research Organisation (ISRO).
