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Gemini/HST survey shows building-blocks in evolution of massive galaxy
A study of the Universe's most massive galaxy clusters has shown that
mergers play a vital role in their evolution.
Astronomers at Oxford University and the Gemini Observatory used a
combination of data from the twin Gemini Telescopes, located in Hawaii
and Chile, and the Hubble Space Telescope (HST) to study populations of
stars in the Universe's most massive galaxy clusters over a range of
epochs -- the earliest being half the age of the Universe. The HST
images were used to map the light distribution of the galaxies in the
cluster. Data from the Gemini Multi-Object Spectrograph allowed the team
to analyse the light from galaxies to determine their masses, ages and
"We still don't have a clear picture of how galaxies develop over the
history of the Universe. The strength of this study is that we are able
to look at galaxy clusters over a range of epochs," said Dr Jordi Barr
of Oxford University, who is presenting some of the first results of the
Gemini/HST Galaxy Cluster Project at the RAS National Astronomy Meeting
on 5th April.
Galaxy clusters contain the most massive galaxies in the Universe. Until
recently, astronomers believed that all galaxies in the centres of
clusters formed rapidly and then aged without any further changes to
their structure in a process known as "Passive Evolution". Results from
the Gemini/HST Galaxy Cluster Project now show that this cannot be the
Dr Barr explained, "When we're looking at the most distant galaxy
clusters, we are looking back in time to clusters that are in early
stages of their formation. The young galaxies in distant clusters appear
to be very different from those in the mature clusters that we see in
the local Universe. We found the earliest galaxy clusters have a huge
variation in the abundances of elements such as oxygen and magnesium,
whereas the chemistry of galaxies in the sample of closer clusters
appears to be much more homogenous. This difference in chemistry proves
that the clusters must actively change over time. If the galaxies in the
old clusters have acquired a complete 'set' of elements, it's most
likely that they have formed from the mergers of several young galaxies."
The group found that the star-formation in galaxies is dependent on mass
and that in lower mass galaxies star-formation continues for longer. The
most massive galaxies in clusters appear to have formed all their stars
by the time the universe is just over a billion years old, whereas the
lower mass galaxies finish forming their stars some 4 billion years later.
"We see the effects of star-formation in low mass galaxies but are
unsure about why it's happening. It's possible that star-formation can
be shut down very rapidly in dense environments and that the lower mass
galaxies are recent arrivals that are forming stars over a longer period
outside the cluster, then falling in. But we are still speculating ..."
said Dr Barr
The group's observations of merging galaxy clusters showed that a large
proportion of the galaxies in those clusters have undergone recent
bursts of star formation. This indicates that star formation may be
triggered if galaxies are thrown, during the course of a merger, into
contact with the gaseous medium pervading the cluster.
Future observations are planned at X-ray wavelengths to study the
interactions between galaxies and the distribution and temperature of
the surrounding gas.
Hubble Space Telescope image of Abell 1689, one of the galaxy clusters
used in sample for the study. © NASA, N. Benitez (JHU), T. Broadhurst
(Racah Institute of Physics/The Hebrew University), H. Ford (JHU), M.
Clampin (STScI), G. Hartig (STScI), G. Illingworth (UCO/Lick
Observatory), the ACS Science Team and ESA
Royal Astronomical Society's National Astronomy Meeting
The 2006 RAS National Astronomy Meeting is hosted by the University of
Leicester. It is sponsored by the Royal Astronomical Society, the UK
Particle Physics and Astronomy Research Council (PPARC), the University
of Leicester and the National Space Centre, Leicester.
Dr Jordi Barr
Denys Wilkinson Building
Oxford OX1 3RH, UK
Tel: +44 (0)1865 273 299
Fax: +44 (0)1865 273 390