Wide field image of PGC 6240 (ground-based image)
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The beautiful, petal-like shells of
galaxy PGC 6240 are captured here in intricate detail by the NASA/ESA
Hubble Space Telescope, set against a sky full of distant background
galaxies. This cosmic bloom is of great interest to astronomers due to
both its uneven structure, and the unusual clusters of stars that orbit
around it — two strong indications of a galactic merger in the recent
past.
PGC 6240 is an elliptical galaxy that resembles a pale rose in the
sky, with hazy shells of stars encircling a very bright centre. Some of
these shells are packed close to the centre of the galaxy, while others
are flung further out into space. Several wisps of material have been
thrown so far that they appear to be almost detached from the galaxy
altogether.
Astronomers have studied PGC 6240 in detail due to this structure,
and also because of its surrounding globular clusters — dense, tightly
packed groups of gravitationally bound stars that orbit galaxies. Over
150 of these clusters orbit our own galaxy, the Milky Way, all composed
of old stars.
All the globular clusters around a certain galaxy form at
approximately the same time, giving them all the same age. This is
echoed within the clusters — all the stars within a single cluster form
at around the same time, too. Because of this, most galaxies have
cluster populations of pretty similar ages, both in terms of overall
cluster, and individual stars. However, PGC 6240 is unusual in that its
clusters are varied — while some do contain old stars, as expected,
others contain younger stars which formed more recently.
The most likely explanation for both the galaxy’s stacked shell
structure and the unexpectedly young star clusters is that PGC 6240
merged with another galaxy at some point in the recent past. Such a
merger would send ripples through the galaxy and disrupt its structure,
forming the concentric shells of material seen here. It would also
ignite a strong burst of star formation in the galaxy, which would then
trigger similar activity in nearby space — leading to the creation of
new, younger globular clusters around PGC 6240.
PGC 6240 is an elliptical galaxy in the southern constellation of
Hydrus (The Water Snake). Also visible in this region are numerous
background galaxies, speckled across the sky behind PGC 6240. Even
though these bodies are at such vast distances from us, it is possible
to make out the structure of many of the galaxies, especially the small
spirals that stand out colourfully against the dark sky.
A version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Judy Schmidt.
Notes
Hidden Treasures is an initiative
to invite astronomy enthusiasts to search the Hubble archive for
stunning images that have never been seen by the general public. The
competition is now closed, and the list of winners is available here.
More information
The Hubble Space Telescope is a project of international cooperation between ESA and NASA.
Links
Contacts
Richard HookESA/Hubble acting Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Email: rhook@eso.org
