Initial results from research carried out as part of the Atlas3D project on two elliptical galaxies could, if they are confirmed, call into question the current model of the formation of galaxies.

An international team of astronomers, including in particular CNRS, CEA, the CFHT, the Observatoire de Lyon, ENS de Lyon and Claude Bernard Lyon 1 et Paris-Diderot universities, are due to publish their initial observations in the journal Monthly Notices of The Royal Astronomical Society.

Current models explain elliptical galaxies as "dead" galaxies: the relative age of their stars ranges between 7 and 10 billion years and the observed lack of gas precludes the formation of new stars. However, a completely different history is suggested by the images of two galaxies obtained by the MégaCam camera of the Canada-France-Hawaii Telescope (CFHT, CNRS/CNRC/University of Hawaii).

The researchers,* all members of an international Atlas3D team, have shown that these two elliptical galaxies were formed from the "merger" of two large spiral galaxies, just 1 to 3 billion years ago. During this major event, part of the matter of the galaxies in "collision" was ejected and formed stellar debris. The filaments of gas and stars, which have been detected by the CFHT, form two long tails on either side of the galaxy, extending over more than one million light years (more than 10 times the span of the Milky Way). It is the largest stellar structure ever detected, although it has not been revealed before due to the low surface brightness and considerable spread of its filaments. These filamentary structures were formed during encounters between spiral galaxies, by a gravitational mechanism similar to that of oceanic tides, hence their name of "tidal tails."

The Atlas3D team is conducting a deep optical imaging program on a hundred or so other nearby elliptical galaxies. If the results obtained on these first two galaxies are confirmed and if such extended stellar structures prove to be frequent, the standard model of the formation of elliptical galaxies needs to be revised.

*In France, the laboratories involved are the Laboratoire Astrophysique, Instrumentation et Modélisation (CNRS/CEA/Université Paris Diderot), the Centre de Recherche Astrophysique de Lyon (CNRS/ENS de Lyon/Université Lyon1), working in collaboration with the CFHT (Canada-France-Hawaii Telescope).