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Although the gas filaments in which galaxies are born have long been predicted by cosmological models, until now we have had no actual images of such objects. Now, for the first time, various filaments of the ‘cosmic web’ have been directly observed using the MUSE instrument (1) installed on ESO’s Very Large Telescope in Chile. These observations of the early Universe, 1 to 2 billion years after the Big Bang, point to the existence of a multitude of dwarf galaxies hitherto unsuspected. Carried out by an international collaboration led by the Center de Recherche Astrophysique de Lyon (CNRS / Université Lyon 1 / ENS de Lyon), with the participation also of the Lagrange laboratory (CNRS / Université Côte d’Azur / Observatoire de la Côte d’Azur ) (2), the study is published on March 18, 2021 in the journal Astronomy and Astrophysics.
The filamentous structure of hydrogen gas in which galaxies form, known as the cosmic lattice, is one of the main predictions of the Big Bang model and galaxy formation. Until now, everything that was known about the web was limited to a few specific regions, particularly in the direction of quasars, whose powerful radiation acts like the headlights of a car, revealing clouds of gas along the line of sight. . However, these regions are not very representative of the entire filament network where most galaxies, including our own, were born. Direct observation of the dim light emitted by the gas that forms the filaments was a holy grail that has now been achieved by an international team led by Roland Bacon, CNRS researcher at the Center de Recherche Astrophysique de Lyon (CNRS / Université Lyon 1 / ENS of Lyon).
The team took the bold step of pointing ESO’s Very Large Telescope, equipped with the MUSE instrument coupled to the telescope’s adaptive optics system, at a single region of the sky for more than 140 hours. Together, the two instruments form one of the most powerful systems in the world. The selected region is part of Hubble’s ultra-deep field, which until now was the deepest image of the cosmos ever obtained. However, Hubble has now been overtaken, as 40% of the galaxies discovered by MUSE have no counterpart in the Hubble images.
After meticulous planning, it took eight months to carry out this exceptional observation campaign. This was followed by a year of data processing and analysis, which for the first time revealed the light from the hydrogen filaments, as well as images of various filaments as they were between one and two billion years after the Big Bang, a period key to understanding how galaxies formed from gas in the cosmic web. However, the biggest surprise for the team was when the simulations showed that the gas light came from a hitherto invisible population of billions of dwarf galaxies that generated a large number of stars (3). Although these galaxies are too faint to be detected individually with current instruments, their existence will have important consequences for models of galaxy formation, with implications that scientists are only beginning to explore.
Grades:
(1) MUSE, which stands for Multiple Unit Spectroscopic Explorer, is a 3D spectrograph designed to explore the distant Universe. The instrument was built by the Center de Recherche Astrophysique de Lyon (CNRS / Université Claude Bernard-Lyon 1 / ENS de Lyon).
(2) Other French laboratories involved: Laboratoire d’Astrophysique de Marseille (CNRS / Aix-Marseille University / CNES), Research Institute for Astrophysics and Planetology (CNRS / Université Toulouse III – Paul Sabatier / CNES).
(3) Until now, the theory predicted that the light came from diffuse cosmic ultraviolet background radiation (very weak background radiation produced by all galaxies and stars) which, by heating the gas in the filaments, makes them glow.
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