Découverte d’une galaxie ultraviolette «extrême»

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Image taken by the Indian satellite AstroSat in the ultraviolet range. The zoom

Image taken by the Indian satellite AstroSat in the ultraviolet range. The zoom shows the AUDFs01 galaxy as seen by the Hubble Space Telescope (left) and AstroSat (right). (Image: UNIGE)

The discovery of the first galaxy emitting "extreme" ultraviolet radiation could help to better understand how the cosmic era known as the "Dark Ages" came to an end more than 13 billion years ago.

Thanks to the Indian satellite AstroSat, an international team, including astronomers from the University of Geneva, has detected the first galaxy, called AUDFs01, emitting "extreme" ultraviolet radiation, i.e. highly energetic (with a wavelength of about 60 nanometres, or billionths of a metre). Ultraviolet" galaxies are rare (only a few dozen are known, all emitting less energetic photons than AUDFs01), but they can shed light on a very early and little-known period in the history of the cosmos: the end of the "dark ages". More than 13 billion years ago, the first stars and galaxies appeared whose ultraviolet radiation was responsible for the ionisation of intergalactic gases and thus for the fact that the Universe became transparent and is still transparent today. As the ionising photons of primordial galaxies are out of reach, astronomers have to turn to closer analogue ultraviolet galaxies, such as AUDFs01, to build their simulation models of this reionisation episode. This discovery can be read in the journal Nature Astronomy.

"Some time after the Big bang, the Universe went through a very dark era, called the ’Dark Ages’," explains Anne Verhamme, professor at the Astronomy Department of the Faculty of Science of the University of Geneva. During this period of a few hundred thousand or hundreds of millions of years, the Universe is black, filled with neutral gas, essentially hydrogen, the simplest and most abundant element in nature. Then, about 13 billion years ago, a radical transition takes place. The first stars and galaxies appear and, little by little, their radiation ionises all the gas atoms in intergalactic space, i.e. it tears off their electrons. The Universe becomes luminous and transparent. This phase is called re-ionisation. And our study should contribute to understanding, by bringing new data to simulation models, how this episode took place, how long it lasted, etc.".

Stellar radiation almost impossible to detect

Only the most massive stars emit radiation energetic enough to break hydrogen atoms. The problem is that this radiation is almost always absorbed by the surrounding hydrogen and therefore difficult to observe directly. As a result, astronomers are forced to rely on theoretical predictions to determine the energy distribution of the ionising photons emitted by such stars. Unfortunately, these predictions differ greatly in the so-called extreme ultraviolet range, especially at wavelengths shorter than 80 nanometres (or billionths of a metre).

Silent primordial galaxies

"It is assumed that the first massive stars that appeared at the end of the Dark Ages must have emitted enough ionising radiation for some of them to escape from primordial galaxies and ionise the intergalactic medium," says Daniel Schaerer, a professor in the Department of Astronomy. But we will never be able to verify this scenario through direct observations. Indeed, the probability that one of these ionising photons, emitted by such a distant galaxy, will reach our telescopes without encountering a hydrogen atom along the way is zero".
To try to find out more, scientists must therefore turn to nearby galaxies, analogous to primordial galaxies, which would emit enough ionising photons for some to escape and reach Earth. The problem is that the vast majority of these nearby galaxies are opaque to ionising photons.

A few dozen galaxies

After decades of research, only a few dozen ultraviolet galaxies have been discovered very recently (all after 2016). They are either very close or much further away, two distances that simply correspond to the detection limits of the devices available on the Earth’s surface and on board current satellites.
"Launched in 2015, AstroSat, India’s first astronomical space observatory, made it possible for the first time to search for ultraviolet galaxies at intermediate distances thanks to its detectors sensitive to a wide range of ultraviolet light," explains Kanak Saha, a professor at the Inter-University Centre for Astronomy and Astrophysics in India and the first author of the article.

"This is how we discovered the first galaxy emitting in the extreme ultraviolet range (with a wavelength of 60 nanometres)," says Charlotte Simmonds, a researcher at the Department of Astronomy and Astronomy at the Inter-University Centre for Astronomy and Astrophysics in India and the first author of the article.