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Leo's gas ring explained
by galaxy collision

Posted: 1 July 2010

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The nature of a giant gas ring that loops around the Leo galaxy group has been deciphered as the remains of a violent galactic smash that took place over one billion years ago.

The Leo ring. This image is taken in the optical domain with the distribution of the gas in HI in yellow-orange. The thumbnails on the right show three of the dense areas of the ring with their optical counterparts. Image: CFHT/Astron - P.A. Duc.

The Leo ring is made up of cold gas and spans a diameter some 650,000 light years across. Its origin has mystified astronomers since its discovery in the 1980s, but last year detection of metals in the gas led astronomers to believe that the ring was composed of primordial gas. According to theory, the accretion of cold, primordial gas plays a key role in the early stages of galaxy formation, but is characterised by the fact it has never been part of a galaxy before, and does not possess the conditions necessary to spawn stars.

Simulation showing the collision of two galaxies to create the Leo ring. Credit: CEA - Léo Michel-Dansac (CNRS CNRS/INSU Université Lyon 1).

Now, thanks to observations using the Canada-France-Hawaii Telescope, astronomers have detected for the first time an optical signature within the densest part of ring. Corresponding to regions containing massive young stars, this observation rules out the primordial nature of the gas.

Combining the observations with computer simulations, the scientists arrived at the conclusion that the ring is instead a relic of a dramatic collision that occurred between two galaxies just over one billion years ago. The two culprits have been identified as NGC 3384, currently located in the centre of the Leo Group, and massive spiral galaxy M96, situated towards the groups edge.