A missing part of the matter of the Universe found thanks to the MUSE instrument

• Galaxies exchange matter with their external environment thanks to galactic winds.
• The MUSE instrument of the Very Large Telescope has allowed for the first time to map a galactic wind at the origin of exchanges between a galaxy and a nebula.
• This observation allowed to detect a part of the missing matter of the Universe.

Galaxies, islands of stars in the Universe, have ordinary matter, called baryons, made up of elements from the periodic table, and dark matter, of still unknown composition. One of the major problems in understanding the formation of galaxies is that about 80% of the baryons [Baryons are particles made of three quarks, like protons and neutrons. They make up atoms and molecules and all visible structures in the observable universe (stars, galaxies, clusters of galaxies, etc.). The "missing" baryons, unobserved until now, are to be distinguished from the dark matter, composed of non-baryonic matter, of unknown nature. The latter are missing [sic] composing the normal matter. According to the models, they would be sent out of the galaxies in the inter-galactic space, thanks to the galactic winds coming from the explosion of stars.

Thanks to the MUSE instrument, an international team [[Scientists from Saint Mary’s University in Canada, the Potsdam Institute of Astrophysics and the University of Potsdam in Germany, the University of Leiden in the Netherlands, the University of Geneva and the Swiss Federal Institute of Technology in Zurich in Switzerland, the Interuniversity Centre for Astronomy and Astrophysics in India and the University of Porto in Portugal. In France, a study conducted by researchers from the CNRS and the Université Claude Bernard Lyon 1 has succeeded in mapping in detail a galactic wind at the origin of exchanges between a young galaxy in formation and a nebula (in other words, a cloud composed of gas and interstellar dust).

The team chose to observe the Gal1 galaxy because of the presence of a nearby quasar, a true "beacon" for the scientists that directed them to the study area. They also intended to observe a nebula around this galaxy. However, the success of this observation was at first uncertain, because the brightness of the nebula was unknown.

Finally, the perfect positioning of the galaxy and the quasar, as well as the discovery of gas exchanges due to galactic winds, made it possible to draw up a unique map. This made possible the first observation of the formation of a nebula that is simultaneously in emission and absorption of magnesium, a part of the missing baryons of the Universe, with the galaxy Gal1.

This type of normal matter nebula is known in the near Universe, but their existence for young galaxies in formation was only supposed.

The scientists have discovered a part of the missing baryons of the Universe, confirming that 80 to 90% of the normal matter is located outside the galaxies. This observation will thus make it possible to supplement the models of evolution of the galaxies.

Bibliography

MusE GAs FLOw and Wind (MEGAFLOW) – VIII. Discovery of a Mg II emission halo probed by a quasar sightline. Johannes Zabl, Nicolas F. Bouché, Lutz Wisotzki, Joop Schaye, Floriane Leclercq, Thibault Garel, Martin Wendt, Ilane Schroetter, Sowgat Muzahid, Sebastiano Cantalupo, Thierry Contini, Roland Bacon, Jarle Brinchmann et Johan Richard. MNRAS, le 16 septembre 2021. https://doi.org/10.1093/mnras/stab2165