PUNE A team of international astronomers led by the scientists from the Inter-University Centre for Astronomy and Astrophysics (Iucaa) in Pune has laid claim to the discovery of UV light emanating from a galaxy called AUDFs01, which, according to Iucaa, is 9.3 billion light-years away from Earth.

The light was seen using “AstroSat”, India’s first multi-wavelength satellite.The multi-wavelength satellite has five unique X-ray and ultraviolet telescopes working in tandem. AstroSat was launched on September 28, 2015, by the Indian Space Research Organisation (ISRO) and has onboard the UltraViolet Imaging Telescope (UVIT).

The discovery was made by a team, led by Dr Kanak Saha, associate professor at Iucaa and the paper has been published on Monday by Nature Astronomy.

The team comprises scientists from Switzerland, France, USA, Japan and the Netherlands, apart from India.

Saha and his team observed the galaxy which is located in the “Hubble Extreme Deep field”, through AstroSat. These observations lasted for 28 hours in October 2016, but it took two years since then to carefully analyse the data to ascertain that the emission is indeed from the said galaxy.

Since UV radiation is absorbed by Earth’s atmosphere, it has to be observed from space.

Nasa’s Hubble Space Telescope (HST), significantly larger than AstroSat, has not detected any UV emission from this galaxy because it is too faint.

“AstroSat or UVIT was able to achieve this unique feat because the background noise in the UVIT detector is much less than the ones on HST,” said Dr Saha.

The 38-cm diameter UVIT, which is capable of simultaneous imaging in far and near-ultraviolet bands with a wide field of view, was developed by teams from IIA, IUCAA, and TIFR from India, and CSA of Canada, under the leadership of Shyam Tandon, ex-emeritus professor, Iucaa.

“We knew 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. This discovery of AUDFs01 by AstroSat establishes that there is hope and perhaps, this is the beginning.” added Dr Saha.

“The excellent spatial resolution and high sensitivity is a tribute to the hard work of UVIT’s core team over a decade, which is the key to the detection of this very weak source,” said Prof Tandon.

“With UVIT observation, AUDFs01 became the first example of a galaxy with clumpy morphology and leaking ionising radiation at 60 nanometers. AUDFs01 offers the first observational constraint in this extreme ultraviolet regime where stellar models are the most discrepant; with further detections, AstroSat will allow us to refine our scenario of cosmic reionisation,” said co-author, Dr Anne Verhamme, professor at the Geneva Observatory, Switzerland.

AUDFs01 is in the middle of a redshift range (0.4 to 2.5) where previously no similar sources were detected. The galaxy is not only bridging the gap between the low and high redshift regime at present, but it is also the beginning of a new exploration of star-forming galaxies at extreme-UV wavelength.

One can use large-aperture, ground-based telescopes like Keck, VLT, and Subaru for observations of ionising photons in the universe of redshift larger than 2.5. But below this redshift, AstroSat becomes a unique facility.

“Indeed, UVIT will play an important role in revealing insights to the epoch of reionisation,” said co-author Dr Abhishek Paswan, a postdoctoral fellow at Iucaa.

Dr Somak Raychaudhury, director, Iucaa said, “This is a very important clue as to how the dark ages of the Universe ended and how there was light in the Universe. We need to know when this started, but it has been very hard to find the earliest sources of light. I am very proud that my colleagues have made such an important discovery.”