AT THE SMITHSONIAN

Strippers in Space: New Study Sheds Light on Galaxy Formation

August 12, 2009

Modern cosmology—the study of stars, not to be confused with the art of applying makeup—is trying to figure out how dwarf spheroidal galaxies form and a new study may help to solve that mystery. If bits of that first sentence made your eyes cross, let's go over a few terms before moving on. Dwarf spheroidal galaxies are small, round-ish galaxies that are very faint and contain few stars in relation to their total mass. This type of galaxy seems to be largely made up of dark matter—a mysterious, invisible substance that exhibits a gravitational influence on other celestial bodies. This is the stuff that composes a considerable chunk of outer space.

Scientists have tossed around theories about how dwarf spheroidals may have formed in relation to large galaxies, like the Milky Way—but those ideas fail to account for dwarf spheroidals that exist in the more remote parts of the universe, and aren't associated with large galaxies.

Elena D'Onghia of the Harvard-Smithsonian Center for Astrophysics, along with her team, explored other possibilities using computer simulations. Their main question was: since a dwarf spheroidal galaxy has so few stars, where did all the stars go? They found that galactic hook ups—such as an encounter between two dwarf galaxies or an encounter between a dwarf galaxy and the Milky Way—can begin a process dubbed "resonant stripping."

"Like in a cosmic dance," D'Onghia explains, "the encounter triggers a gravitational resonance that strips stars and gas from the dwarf galaxy, producing long visible tails and bridges of stars." Essentially, these galaxies are cosmic dancers performing a stellar striptease.

Although this team's conclusions are drawn from computer-generated simulations, there is evidence up in the heavens that supports their work—notably the bridge of stars that exists between the Leo IV and Leo V dwarf spheroidal galaxies which may have resulted from resonant stripping.