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Leo II: Young at heart

A team of astronomers has discovered that Leo II, an old dwarf galaxy, has a juvenescent heart.
Provided by the Subaru Telescope Facility, Hilo, Hawaii
The field of view this false-color image of Leo II (composed from V and Ic band images) is 26.67 by 26.67 square arcminutes. Integration times are 3000 sec in V band and 2400 sec in Ic band, respectively. North is up, east is left.
Subaru Telescope
November 29, 2007
Using the 8.2-meter Subaru Telescope on the summit of Mauna Kea, an observing team of 15 Japanese astronomers conducted a thorough inspection of a galaxy building block. They found that a small galaxy named Leo II largely consists of older stars, a sign of survival through galactic cannibalism under which massive galaxies, such as the Milky Way, consume smaller galaxies to attain their extensive size. Researchers also found younger stars in the galactic center of Leo II.

Leo II is categorized as a spheroidal dwarf galaxy that resides approximately 760,000 light years away from our Milky Way. Even though this is one of the galaxies closest to us, it is very faint, small, and difficult to make detailed observations.

The astronomers used the 80-megapixel Suprime-Cam camera on the Subaru Telescope to conduct their research because its wide-field coverage and good image quality are ideal for investigating stellar contents of such galaxies.

Over 2 nights, 90 minutes of exposures were taken and 82,252 stars were detected down to a visible magnitude of 26 (the human eye cannot see below magnitude 6). The group reported that the data set contained excellent results providing critical information on the properties of the stars within Leo II.

The scientists found red giant stars throughout Leo II, toward the center as well as the outer areas. The results further showed that the stars in the outer portions of the galaxy are metal deficient, that is, older than towards the inner areas. In addition, observations identified some remnant of globular clusters towards the eastern edge of Leo II that may have experienced tidal disruption; their violent history is yet to be explained.

The astronomers concluded that major star formation occurred at a modest rate in Leo II about 8 billion years ago, and that star formation started from the outside toward the center stopping approximately 4 billion years ago, except for the very center of the galaxy where younger stars are found.
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