Cosmic Dance Helps Galaxies Lose Weight

July 29, 2009
This simulation illustrates the resonant stripping process. Stars of a dwarf galaxy (at the bottom) orbiting around a larger system (at the top) are stripped off by gravity, forming long tails of stars. Credit: Elena D'Onghia (CfA)

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 set up 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. Credit: Elena D'Onghia (CfA)
( -- A study published this week 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 , 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 , 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 , and understanding how they formed is a principal goal of modern cosmology," said lead author Elena D'Onghia of the Harvard-Smithsonian Center for Astrophysics (CfA).

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 , 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 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.

The paper describing these results was authored by Elena D'Onghia, Gurtina Besla, Thomas J. Cox, and Lars Hernquist, all of the CfA.Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

Provided by Harvard-Smithsonian Center for Astrophysics (news : web)

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4 / 5 (1) Jul 29, 2009
A paper on the discovery of Leo V (and possible connection to Leo IV)can be found here: . Additional info on the dynamics of Leo V are available here: . Seems like an intriguing idea. Any links available to a preprint of the paper referenced in the article?
5 / 5 (3) Jul 30, 2009
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.
'Resonant Stripping' seems to be the process by which ordinary matter like stars are preferentially pulled from the dwarf galaxy, leaving the dark matter behind.

I wonder if there is any pre-existing evidence of this 'resonant stripping'? Or whether this is just an imaginative idea on the part of the computer modellers to arrive at a desired scenario?

The original article is at -

For a more informative and less one-sided report see
5 / 5 (1) Jul 30, 2009
A pity that 'Dark Matter' articles are not cross-referenced on PhysOrg or, at least, catalogued together as a sub-topic of 'Astronomy'. With so much contradictory 'evidence' in this debate, it is worthy of its own news feed.
not rated yet Jul 30, 2009
Thanks, smiffy, for both links regarding this work. The paper states that their model makes two predictions, 1) that dwarf spheroidal galaxies produce tails or shells around their more massive partners and 2) dwarf spheroidals should appear the same in different extragalactic systems and galaxy clusters. Several dwarf spheroidals orbiting the Milky Way Galaxy are known to exhibit long tails, both proceeding and following the dwarf itself. Other stellar streams presumed to be associated with dwarf galaxies are observed in other galaxies as well (NGC 5907, NGC 1097, UGC 10214). The paper also mentions that similar dwarf spheroidals are observed in the Perseus galaxy cluster (Abell 426). The Science News article brings up some good questions about various aspects of this theory, the most compelling I found to be if resonant stripping can account for the majority of the observed dwarf spheroidal population(s). It should be noted that these new low luminosity dwarfs recently discovered surrounding our galaxy have been poorly studied, due in part to the difficulty of making the observations themselves. As more data on them is acquired, astronomers will be in a better position to test this proposed scenario. Perhaps resonant stripping may represent one of several mechanisms leading to stellar stripping.
1 / 5 (4) Jul 30, 2009

Cosmic observations and efforts to explain them without understanding basic forces between nucleons generates only fictional astrophysics and cosmology.

See: http://www.physor...732.html

With kind regards,
Oliver K. Manuel
not rated yet Aug 08, 2009
A pity that 'Dark Matter' articles are not cross-referenced on PhysOrg or, at least, catalogued together as a sub-topic of 'Astronomy'. With so much contradictory 'evidence' in this debate, it is worthy of its own news feed.

Ray, doing a search on 'dark matter' will at least give you a list of historical articles - there's a awful lot. The search function is hidden away at the bottom of each page.

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