How Strange, Small Galaxies Lost Their Stars

Close encounters of the galactic kind may explain the existence of an unusual type of dwarf galaxy, a new study suggests.

So-called dwarf spheroidal galaxies are small and very faint, containing few stars relative to their total mass.

These star-deprived galaxies appear to be made mostly of dark matter ? an elusive form of matter detectable only by its gravitational influence. Dark matter outweighs normal matter by a factor of five to one in the universe as a whole.

Astronomers have found it difficult to explain the origin of dwarf spheroidal galaxies.

"These systems are 'elves' of the early universe, and understanding how they formed is a principal goal of modern cosmology," said study author Elena D'Onghia of the Harvard-Smithsonian Center for Astrophysics (CfA).

Previous theories have required that dwarf spheroidals orbit near large galaxies like the Milky Way ? but this doesn't explain how the dwarfs that have been observed in the outskirts of the "Local Group" of galaxies (which includes the Milky Way) could have formed.

D'Onghia and her colleagues used computer simulations to examine two formation scenarios:

• An encounter between two dwarf galaxies far from giants like the Milky Way, with the dwarf spheroidal later accreted into the Milky Way.
• An encounter between a dwarf galaxy and the forming Milky Way in the early universe.

The team found that the galactic encounters excite a gravitational process which they term "resonant stripping," leading to the removal of stars from the smaller dwarf over the course of the interaction and transforming it into a dwarf spheroidal.

"Like in a cosmic dance, the encounter triggers a gravitational resonance that strips stars and gas from the dwarf galaxy, producing long visible tails and bridges of stars," D'Onghia said

"This mechanism explains the most important characteristic of dwarf spheroidals, which is that they are dark-matter dominated," said co-author Gurtina Besla, also of the CfA.

The long streams of stars pulled off by gravitational interactions should be detectable. For example, the recently discovered bridge of stars between Leo IV and Leo V, two nearby dwarf spheroidal galaxies, may have resulted from resonant stripping.

The team's findings were detailed in the July 30 issue of the journal Nature.

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