Letters from Iwo Jima

ScienceDaily (July 29, 2009) — A study published July 30 in the journal Nature offers an explanation for the origin of dwarf spheroidal galaxies. The research may settle an outstanding 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, which outweighs normal matter by a factor of five to one in the universe as a whole.

D’Onghia and her colleagues used computer simulations to examine two scenarios for the formation of dwarf spheroidals: 1) an encounter between two dwarf galaxies far from giants like the Milky Way, with the dwarf spheroidal later accreted into the Milky Way, and 2) 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.