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Are elliptical galaxies much younger than previously thought?

The new age estimate is based on the presence of ultra-faint filaments in the distant outskirts of two galaxies.
NGC5557
Galaxy NGC 5557 exhibits extremely extended and faint tidal streams spanning more than 1.2 million light-years from left to right on this image from the MegaCam mounted on the Canada-France-Hawaii Telescope. Image credit: P.-A. Duc 2011 (c) CEA/CFHT
The standard model for elliptical galaxy formation is challenged by a new result uncovered by an international team of astronomers from the Atlas3D collaboration. The first results from the group’s study on two elliptical galaxies exhibiting features characteristic of a fairly recent merger, suggest they are 5 times younger than commonly thought.

The accepted belief on the mass assembly history of massive elliptical galaxies based on their stellar population leads to an age between 7 and 10 billion years old. A different story is shaping up based on ultra-deep images of two galaxies observed with the MegaCam camera mounted on the Canada-France-Hawaii Telescope (CFHT).

Elliptical-galaxies
A sample of elliptical galaxies from the Atlas3D survey's current collection, all showing clear signs of a recent collision. Image credit: P.-A. Duc 2011 (c) CEA/CFHT
The astronomers established that the formation of the two elliptical galaxies, NGC 680 and NGC 5557, originated from a merger of two giant spiral galaxies that took place only 1 to 3 billion years ago. The age estimate is based on the presence of ultra-faint filaments in the distant outskirts of the galaxies. These features called tidal streams are typical residuals from a galaxy merger. They are known not to survive in this shape and brightness for more than a few billion years, hence the new age estimate of the resulting elliptical galaxies. These structures were detected for the first time thanks to an imaging technique boosting the capabilities of CFHT's wide-field optical imager MegaCam.

The Atlas3D team conducts a systematic survey of more than 100 nearby elliptical galaxies. If the current result based on the first two galaxies is confirmed on the larger sample, i.e. if faint extended features are frequently detected, the standard model for elliptical galaxies formation should be revisited.

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