from UniverseToday Website
are distorted into elongated structures consisting of stars,
which are known as tidal streams, as shown in this artist's impression.
Credit: Jon Lomberg
Globular clusters are generally some of the oldest structures in our galaxy.
Many of the most famous ones formed around the same time as our galaxy, some 13 billion years ago.
However, some are distinctly younger. While many classification schemes are used, one breaks globular clusters into three groups: an old halo group which includes the oldest of the clusters, those in the disk and bulge of the galaxy which tend to have higher metallicity, and a younger population of halo clusters.
The latter of these provides a bit of a problem since the galaxy should have settled into a disk by the time they formed, depriving them of the necessary materials to form in the first place.
But a new study (The
Globular Cluster System of The Milky Way - Accretion in A
Cosmological Context) suggests a solution that’s not of
The young halo extends to as much as 120 kiloparsecs (400 thousand light years) while the old halo clusters tend to lie within 30 kiloparsecs (100 thousand light years).
Additionally, the young clusters don’t
appear to be rotating with the disk of the galaxy whereas the old
halo slowly orbits in the same direction as the disk.
This finding suggests that this population of globular clusters is a relic of cannibalized galaxies.
Even more interesting is that, while these objects are younger than the distinctly “old” population, there is still a large variation in their ages. This implies that this plane wasn’t created by the accretion of one, or even a few minor galaxies, but a consistent feeding of small galaxies onto the Milky Way for much of the history of the universe, and all from the same direction.
Studies of the distribution of
satellites around our nearest major neighbor,
M31, the Andromeda galaxy, has
turned up a similar preferred plane, tilted some 59° from its disk.
One of the reasons astronomers believe
our own dark matter halo is so nicely shaped is the way it is
affecting the Sagittarius dwarf galaxy which is slowly being
accreted onto our own. If the dark matter were more wispy, it should
be stretched out in different manners.
In other words, the filament could be a fossil of larger structure before our galaxy formed along which these dwarf galaxies formed and from which these galaxies could have been slowly accreting over the history of the galaxy.