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Cosmic dance helps galaxies lose weight

Scientists found that galactic encounters excite a gravitational process, leading to the removal of stars from the smaller dwarf spheroidal galaxy.
Provided by the Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts
Dwarf galaxies
D'Onghia and her colleagues simulated encounters between galaxies to determine how dwarf galaxies lose much of their stars and gas. Top Row: Interaction between a dwarf galaxy orbiting around a larger galaxy with 100 times its mass. Only the stars are plotted. The upper left panel illustrates the initial setup where the two dwarfs approach one another. The upper middle panel gives the state of the system after 2 billion years, and the upper right panel shows the appearance of the galaxies after 7 billion years. Bottom Row: Shown is the orbit of the same small galaxy (in white) around the Milky Way today (in yellow), which has 10,000 times its mass.
CfA/Elena D'Onghia
July 31, 2009
A study offers an explanation for the origin of dwarf spheroidal galaxies. The research may settle a puzzle in understanding galaxy formation.

Dwarf spheroidal galaxies are small and very faint, containing few stars relative to their total mass. They appear to be made mostly of dark matter — a mysterious substance detectable only by its gravitational influence that outweighs normal matter by a factor of 5-to-1 in the universe as a whole.

Astronomers have found it difficult to explain the origin of dwarf spheroidal galaxies. Previous theories required that dwarf spheroidals orbit near large galaxies like the Milky Way, but this does not explain how dwarfs that have been observed in the outskirts of the Local Group of galaxies could have formed.

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

D'Onghia and her colleagues used computer simulations to examine two scenarios for the formation of dwarf spheroidals: First, an encounter between two dwarf galaxies far from giants like the Milky Way, with the dwarf spheroidal later accreted into the Milky Way; and second, 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 that 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," said D'Onghia.

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