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| An image of the Fornax cluster in
which the new class of ultra compact dwarf galaxies
(UCDs) were found. Images of two of the UCDs are shown
in the insets. Numerical simulations conducted at UNSW
would suggest that they were once bigger galaxies that
were stripped down by tidal forces as they orbited within
the cluster; the resulting trail of debris predicted
by the simulations is superimposed. |
In its pursuit of understanding objects that are many light-years
away, astrophysics inevitably suffers from the “tyranny
of distance”. The distances are so vast that from our
perspective here on Earth, stars are reduced to point sources
of light, galaxies cannot generally be resolved into their
individual stars, and planets around other stars can only
be detected by indirect means (eg. through the motion they
induce or the obscuring effects they have on their parent
star).
Taken to its extreme, it is conceivable that this problem
could cause even galaxies to appear as point sources, particularly
those that are rather small and compact. Consequently, such
objects would be missed in traditional surveys, where galaxies
are identified by their extended and resolved structure.
Indeed the notion of a compact, “star-like”
galaxy is completely absent from all the morphological schemes
used to classify galaxies, for the very reason that none
have been found.
The first clues that such a population of galaxies might
exist were obtained by Dr Michael Drinkwater (formerly of
UNSW, now at Uni of Qld) and his collaborators, when they
used the Two Degree Field (2dF) spectrograph on the 3.9m
Anglo-Australian Telescope to survey the nearby Fornax cluster
of galaxies. This survey was unusual in that it included
all objects, be they point-like, extended, or fully resolved
spiral and elliptical galaxies. Quite surprisingly, seven
of the point-like objects – assumed to be stars in
our galaxy – turned out to be members of the Fornax
cluster, some 60 million light-years away! The question
then was: what were they? Given that they were so faint,
were they actually galaxies, or were they just massive star
clusters associated with the dominant galaxies in the centre
of Fornax?
We demonstrated that these objects are indeed galaxies
in their own right, based on further observations made with
the Keck and VLT 8-10m telescopes and the Hubble Space Telescope.
Remarkably, they are ~10,000 times less luminous than our
galaxy and only ~300 light-years across – a type of
system previously unknown and which led us to dub them “ultra
compact dwarf” (UCD) galaxies.
Through the numerical simulation work of Kenji Bekki, UNSW
has made a critical contribution to this study by providing
crucial insights into what the origin of these UCDs might
be. The most compelling formation mechanism would appear
to be one of galaxy “threshing”, where the tidal
forces acting within the Fornax cluster strip away the outer
envelopes of (the well known) nucleated dwarf galaxies,
leaving behind only their naked compact cores. This scenario
is captured in the accompanying picture, which shows the
Fornax cluster, two of the UCD objects, and a typical orbit
(as indicated by the trail of debris) that leads to this
destruction.
Warrick Couch and Kenji Bekki
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