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A big day for the country as India's Astrosat guides global team of scientists discover one of the earliest galaxies

  • Adrija Ray Chaudhury | Team PresentMirror | Updated: Aug. 27, 2020, 8:29 p.m.

In a major breakthrough, a global team led by scientists of the Inter-University Centre for Astronomy and Astrophysics (IUCAA) has discovered one of the earliest galaxies in extreme ultraviolet light using Astrosat which is India's first multi-wavelength satellite. What makes this exploration even more astounding, is the very fact that the larger and more efficient Hubble Space Telescope was never able to provide any significant evidence in favour of the presence of this galaxy.


AstroSat, India's first multi-wavelength satellite, has detected an extreme ultraviolet (UV) light from a galaxy which is 9.3 billion light-years away from Earth. The satellite has five unique X-ray and ultraviolet telescopes that work simultaneously which has detected extreme UV light from a galaxy called AUDFs01, 9.3 billion light-years away from Earth.

ISSDC Astrosat captures; Image credits: ISSDC

This insurmountable discovery was made by an international team of astronomers led by Dr Kanak Saha, associate professor of astronomy at IUCAA, and was published on August 24 by Nature Astronomy. The team comprises scientists from India, Switzerland, France, USA, Japan and the Netherlands. In an interview with the Indian Express, Dr Saha said that the galaxy is located in the Extreme Deep Field in space, which is host to another 10,000 galaxies. The observations lasted for more than 28 hours in October 2016. But it took nearly two years since then to carefully analyse the data to ascertain that the emission is indeed from the galaxy.


Dr Somak Raychaudhury, director of IUCAA has raised an interesting question to justify the importance of this discovery. Science has previously led to the discovery of particles formed after the Big Bang but they could not find any source of light. This phase is called the Dark Age of the Universe. But there exists no specific clue how this Dark Age ended and light formed in the Universe. Dr Raychaudhury believes that this discovery currently stands at the apotheosis of space technology and is a portentous success as they have been able to capture the oldest source of light.

How was this feat accomplished?

isro Astrosat; Image Credits: ISRO

Since most of the UV radiation is absorbed by the ozone layer in the Earth’s atmosphere, it has to be observed from space. Earlier, NASA’s Hubble Space Telescope (HST), which is significantly larger than the UltraViolet Imaging Telescope (UVIT) on Astrosat, could not detect any UV emission (with energy greater than 13.6 eV) from this galaxy because it was too faint. AstroSat/UVIT was able to achieve this unique feat because the background noise in the UVIT detector is much less than the ones on HST.

Dr Saha and his team knew that it would be an uphill task to convince the international community that UVIT has recorded extreme-UV emission from this galaxy when the more powerful HST has not, which he has also mentioned in various interviews that followed afterwards. However, he does regard this finding as the beginning of everything.

What makes this achievement the touchstone of technological advancement?

A statement issued by IUCAA explained that after the Big Bang, the Universe was a hot soup of subatomic particles (i.e., protons, neutrons, and electrons). Because the universe had the conditions of the core of a star, it had the temperature and pressure to actually fuse hydrogen into helium and other heavier elements. The fusion process generates photons of gamma radiation. In the core of our sun, these photons bounce from atom to atom, eventually making their way out of the core, through the sun's radiative zone, and eventually out into space. This process can take tens of thousands of years. But in the early universe, there was nowhere for these primordial photons of gamma radiation to go. Everywhere was the more hot, dense universe and thus the universe remained dark and opaque.

recombination The era of Recombination; Image Credits: NASA

The universe was continuing to expand, and finally, just a few hundred thousand years after the Big Bang, the universe was finally cool enough for these atoms of hydrogen and helium to attract free electrons, turning them into neutral atoms. This was the moment of first light in the universe, between 240,000 and 300,000 years after the Big Bang, known as the Era of Recombination. The first time that photons could rest for a second, attached as electrons to atoms. It was at this point that the universe went from being totally opaque, to transparent. Following this came the cosmic dark ages - a period of time after the Universe became transparent but before the first stars formed.

timeline Image Credits: NASA

When the first stars formed, it ended the dark ages and started the next epoch in our universe-the epoch of Reionization. Here, the energy pouring out from the young stars and galaxies ionized the hydrogen and helium, splitting the atoms back again in protons and electrons.

Astronomers have been looking for sources that reionized the early universe though they couldn't prosper as observing ionizing radiation from the first celestial objects is next to impossible. The probability that a fraction of these emanating extreme-UV photons from the host galaxy will get caught by a telescope on Earth is practically zero because these photons will be absorbed by the gas in the galaxy or the gas surrounding the galaxy or the matter between the galaxy and us. In spite of all obstacles, how some of these high-energy photons managed to cross all the barriers and reach Earth is indeed a mystery. But at least we are one step forward now.

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