Close
encounters of the galactic kind may explain the existence of an unusual type of
dwarf galaxy, a new study suggests.
So-called dwarf
spheroidal galaxies are small and very faint, containing few stars relative
to their total mass.
These
star-deprived galaxies appear to be made mostly of dark
matter — an elusive form of matter detectable only by its gravitational
influence. Dark matter 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.
"These
systems are 'elves' of the early universe, and understanding how they formed is
a principal goal of modern cosmology," said study author Elena D'Onghia of
the Harvard-Smithsonian Center for Astrophysics (CfA).
Previous
theories have required that dwarf spheroidals orbit near large galaxies like the
Milky Way — but this doesn't explain how the dwarfs that have been observed
in the outskirts of the "Local Group" of galaxies (which includes the
Milky Way) could have formed.
D'Onghia
and her colleagues used computer simulations to examine two formation scenarios:
- An
encounter between two dwarf galaxies far from giants like the Milky Way,
with the dwarf spheroidal later accreted into the Milky Way.
- 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," D'Onghia said
"This
mechanism explains the most important characteristic of dwarf spheroidals,
which is that they are dark-matter dominated," said co-author Gurtina
Besla, also of 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.
The team's
findings were detailed in the July 30 issue of the journal Nature.
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