Dwarf Irregulars and Galaxy Clusters
Galaxies were once viewed as immutable
objects, persisting in their present form for
most of the history of the universe. Over the
last decade, however, astronomers have come to
understand that many galaxies experience cataclysmic
events which can drastically alter their appearance.
Galaxies interact with each other through gravity;
close passages can remove material from one and
add it to another. Outright collisions can merge
two galaxies into one, perhaps even changing one
type of galaxy into another. Smaller galaxies
can be completely pulled apart and destroyed by
the tidal forces exerted by larger galaxies. These
various interactions were probably more common
at early times in the universe when galaxies were
first being assembled, but even at the present
epoch, it is possible to find spectacular examples
of galaxy interactions and even destruction.
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Illustration Credit:
NASA, ESA, Z. Levay and L. Frattare (STScI)
Image Credit: A. Karick and M.
Gregg (LLNL/UC,D) using the Michigan Curtis
Schmidt Telescope at CTIO: digital color
composite/mosaic in B, V and I bands. Image
Courtesy: M. Drinkwater (University
of Queensland). |
Galaxy clusters, where hundreds
or even thousands of galaxies inhabit dense quarters,
provide a busy environment, an excellent place
to look for intense galaxy interactions. Even
the relatively innocent-looking nearby (60 million
light years) Fornax cluster has an example of
cataclysmic galaxy evolution in progress. Fornax
appears to be a quiet place, even a bit dull,
being dominated by old galaxies, ellipticals and
S0's mostly. This appearance is deceiving, however;
X-ray images reveal a more turbulent picture of
a cluster in the late stages of a recent merger
of a sizable subgroup with the main cluster. A
recent closer look at the relative motions of
the galaxies has revealed additional evidence
for not one but two subunits of galaxies colliding
with the main group.
In retrospect, Fornax contains a
prominent visual clue that the cluster is not
entirely serene: NGC1427A, the only gas-rich irregular
galaxy in the cluster, has an unusual "swoosh''
morphology with prominent blue knots of ongoing
star formation. It also has a high velocity relative
to the main cluster (about 600 km/s) and is probably
part of one of the subgroups making its first
passage through the main cluster. NGC1427A is
relatively isolated, though, so its unusual appearance
and vigorous star formation cannot be blamed on
interaction with any particular large companion,
but instead must be caused by the cluster as a
whole. The motions of the stars which make up
NGC1427A are distorted, and part of the galaxy
appears to be breaking away to the north. This
is highly suggestive that NGC1427A will not long
survive as an identifiable galaxy passing through
the cluster, but will be disrupted, spilling its
stars and gas into the intergalactic regions of
the Fornax cluster.
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Such events, now relatively rare
in rich clusters, were probably quite
common when hundreds of galaxies were first being
assembled into
clusters. The stars and gas ripped from countless
galaxies in a
cluster contribute to building up the giant galaxies
found at the
heart of most clusters, and also account for the
recently discovered
ghostly (but substantial) intergalactic stellar
populations.
Astronomers are using the data from
galaxies like NGC 1427A to investigate the star
formation patterns throughout the object, to determine
if there is a relation between the ages of stars
and position within the galaxy. This will help
to understand how the gravitational influence
of the cluster has affected the internal workings
of this galaxy and how this galaxy has responded
to passing through the cluster environment. Because
it has a relatively high velocity with respect
to the main cluster, after NGC1427A is disrupted
in about 1 billion years, its individual stars,
star clusters, and remaining gas will be spread
far and wide throughout Fornax, contributing to
the thin intergalactic sea of stars and the million
degree gas which glows brightly in X-rays.
The disruption of objects like NGC1427A,
and even larger galaxies like our own Milky Way,
are an integral part of the formation and continued
evolution of galaxy clusters. The impending destruction
of NGC1427A provides but a glimpse of early and
much more chaotic times in the Universe.
