In a major breakthrough, a worldwide team led by scientists of the Inter University Centre for Astronomy & Astrophysics (IUCAA) has discovered one among the earliest galaxies in extreme ultraviolet. AstroSat, India First Multi-Wavelength Satellite that has 5 unique X-ray & ultraviolet telescopes working in tandem, has detected extreme UV light from a galaxy called AUDFs01, 9.3 billion light-years faraway from Earth.
The discovery was made by a world team of astronomers led by Dr Kanak Saha, professor of astronomy at IUCAA, and was published on Aug 24 by Nature Astronomy. The team comprises scientists from India, Switzerland, France, USA, Japan & Netherlands.
Dr Saha told The Indian Express that the team observed the galaxy, which is found within the Extreme Deep field, through AstroSat. These observations lasted for quite 28 hours in Oct 2016. But it took nearly 2 years since then to carefully analyse the info to determine that the emission is indeed from the galaxy.
This is a really important clue to how the Dark Ages of the Universe ended and there was light within the Universe, said Dr Somak Raychaudhury, director of IUCAA. “We got to know when this started, but it’s been very hard to seek out the earliest sources of light,” he said.
Since UV radiation is absorbed by Earth’s atmosphere, it’s to be observed from space. Earlier, NASA’s Hubble Space Telescope (HST), which is significantly larger than the Ultra Violet Imaging Telescope (UVIT) on Astrosat, didn’t detect any UV emission (with energy greater than 13.6 eV) from this galaxy because it’s too faint.
“AstroSat/UVIT was ready to achieve this unique feat because the ground noise within the UVIT detector is far but those on HST,” said Dr Saha.
He added, “We knew it might 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’s hope and maybe , this is often the beginning”.
A statement issued by IUCAA explained that after the Big Bang, the Universe was a hot soup of particles (i.e.,protons, neutrons, & electrons). because the universe began to cool, the protons and neutrons began combining into ionized atoms of hydrogen (and eventually some helium). These ionized atoms of hydrogen and helium attracted electrons, turning them into neutral atoms — which allowed light to travel freely for the 1st time, since this light was not scattering off free electrons. The universe was not opaque. But there no stars, & no galaxies, & therefore the Universe was dark.
A while after this, maybe a some of hundred million years after the Big Bang, the dark Age ended when the 1st stars & galaxies formed and therefore the energy pouring out from them ionized the hydrogen and helium, splitting the atoms back again in protons and electrons, — this is often the epoch of reionization.
Astronomers are trying to find sources that reionised the early universe. The standard suspects are the 1st astronomical objects, especially the newborn small galaxies. But observing radiation from these sources is next to impossible. The probability that a fraction of extreme-UV photons escape the host galaxy and are caught by a telescope on Earth is practically zero, because these photons are going to be absorbed by the gas within the galaxy or the gas surrounding the galaxy or the matter between the galaxy and us.
How a number of these high-energy photons manage to cross all the barriers and reach Earth may be a mystery. The absorption within the intergalactic medium is so severe that it’s impossible to obesrve ionizing-photons within the reionisation epoch directly.
“In the later epoch, the intergalactic absorbers decrease and that we have an opportunity to detect such photons, but it’s still look like a lottery,” said co-author Dr Akio Inoue, professor of Waseda University in Japan.