Galaxies Bring Dancing With the Stars to New Level

By Nancy Atkinson - July 30, 2009 09:47 AM UTC | Extragalactic

[/caption] What is small, mysterious, faint, in the process of losing mass, and can dance like crazy? Could it be Marie Osmond? Well, that might be the correct answer in this galaxy, but just on the outskirts of the Milky Way are small, mysterious galaxies called dwarf spheroidal galaxies, and a new study offers an explanation for the origin of these puzzling objects. But can they really dance? Yes, says lead author Elena D'Onghia of the Harvard-Smithsonian Center for Astrophysics.

These 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.

Astronomers have found it difficult to explain the origin of dwarf spheroidal galaxies. Previous theories require 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 D'Onghia. [caption id="attachment_36374" align="aligncenter" width="250" caption="This simulation demonstrates the resonant stripping process. Stars of a dwarf galaxy (bottom) orbiting a larger system are stripped off by gravity. Credit: CfA"]

[/caption] 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.

"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," explained D'Onghia.

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

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.

Source:

Harvard Smithsonian CfA

Nancy Atkinson

Nancy Atkinson

Nancy Atkinson is a space journalist and author with a passion for telling the stories of people involved in space exploration and astronomy. She is currently retired from daily writing, but worked at Universe Today for 20 years as a writer and editor. She also contributed articles to The Planetary Society, Ad Astra (National Space Society), New Scientist and many other online outlets.

Her 2019 book, "Eight Years to the Moon: The History of the Apollo Missions,” shares the untold stories of engineers and scientists who worked behind the scenes to make the Apollo program so successful, despite the daunting odds against it. Her first book “Incredible Stories From Space: A Behind-the-Scenes Look at the Missions Changing Our View of the Cosmos” (2016) tells the stories of 37 scientists and engineers that work on several current NASA robotic missions to explore the solar system and beyond.

Nancy is also a NASA/JPL Solar System Ambassador, and through this program, she has the opportunity to share her passion of space and astronomy with children and adults through presentations and programs. Nancy's personal website is nancyatkinson.com