The MUSE instrument on ESO's Very Large Telescope in Chile has revealed very detailed haloes of neutral hydrogen around distant galaxies. A new result zooms on a few such haloes, one of them forming a large, almost complete ring of light.

This result will be presented by Adelaide Claeyssens (Centre de Recherche Astrophysique de Lyon) at the annual meeting of the European Astronomical Society (EWASS 2019) in Lyon, France, on 25 June.

Galaxies are surrounded by copious amounts of neutral hydrogen in a region extending very far away from their center. This region is very important as it is the location of the exchanges of gas between galaxies and their environment, and a key element to understand galaxy formation and evolution. In distant galaxies, this region can be revealed through the glow of the gas emission in a halo.

The MUSE instrument, in operation on the Very Large Telescope of the European Southern Observatory, is extremely efficient at identifying haloes around almost all distant galaxies it observed, but they are generally too small to show any detail or structure. The new study combined the power of MUSE with the so-called gravitational lensing effect of galaxy clusters.

Adelaide Claeyssens explains: "Indeed, massive clusters have the property to bend light rays passing through their center, as predicted by Einstein. This produces the effect of a magnifying glass: the images of background galaxies are magnified."

The study presents the two most spectacular observations of magnified haloes obtained to date with MUSE; in one case the halo presents the form of a large, almost complete ring of light.

By zooming on such hydrogen haloes surrounding galaxies, it is possible for the first time to study with unprecedented detail how the gas properties vary across the halo. This information is critical for understanding the physical processes at play in the halo, that is, how homogeneous it is and how the gas moves around the galaxy.

This discovery demonstrates the potential of MUSE combined with the lensing effects to study many more such haloes and to acquire a detailed view about galaxy formation in the universe, when it was only a few billion years old.

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