Combining data from the NASA/ESA Hubble Space Telescope and the ESO Very
Large Telescope (VLT), a group of European and American astronomers have
made a major discovery. They have identified a huge number of ‘young’
stellar clusters, in an old elliptical galaxy. For the first time, it has
been possible to identify several distinct periods of star formation in a
galaxy as old as this one. Elliptical galaxies have always been considered
to have undergone one early star-forming period and thereafter to be
devoid of star formation. However, the combination of the best and largest
telescopes in space and on the ground has now clearly shown that there is
more than meets the eye.

Do elliptical galaxies only contain old stars?

One of the challenges of modern astronomy is to understand how galaxies –
large systems of stars, gas and dust – form and evolve. When did most of
the stars in the Universe form? Did this happen at a very early stage,
within a few billion years of the Big Bang? Have a significant number of
the stars we now observe formed much more recently?

Spectacular collisions between galaxies take place all the time,
triggering the formation of thousands or even millions of stars. However,
when looking at the Universe as a whole, most of its stars are found in
elliptical galaxies whose overall appearance has so far led us to believe
that they, and their stars and as well, are old.

These elliptical galaxies do shine with the diffuse, reddish glow normally
associated with stars that are many thousand million years old. However,
what is the underlying mix of stars that produces this elderly appearance?
Could a significant number of much younger stars be ‘hiding’ among the
older ones?

Detailed observations with the world’s premier telescopes have now cast
new light on this central question about the behaviour of some of the
major building blocks of the Universe.

Cosmic paleonthology

To break the stellar ‘cocktail’ in elliptical galaxies down into its
different constituents, a team of European and American astronomers
observed massive stellar clusters in and around nearby galaxies. These
“globular” clusters, so called because of their shape, exist in large
numbers around all observed galaxies and form a kind of ‘skeleton’ within
their host galaxies. These ‘bones’ receive an imprint for every episode of
star formation they undergo. By reading the ages of the globular clusters
in a galaxy, it is possible to identify the past epoch(s) of active star
formation in a galaxy.

Reading the imprints and deducing the distribution of ages of the globular
clusters, astronomers can reveal when many of the stars in elliptical
galaxies formed. This is similar to the way a palaeontologist uses the
skeletons of dinosaurs to deduce information about the era in which they

A surprising discovery

The team combined images of a number of galaxies from Hubble’s Wide Field
and Planetary Camera 2 with infrared images obtained from the multi-mode
ISAAC instrument on the 8.2m VLT Antu telescope at the ESO Paranal
Observatory (Chile). To their great surprise, they discovered that many of
the globular clusters in one of these galaxies, NGC 4365, a member of the
large Virgo cluster of galaxies, were only a few thousand million years
old, much younger than most of the other stars in this galaxy (roughly 12
thousand million years old).

The astronomers were able to identify three major groups of stellar
clusters. There is an old population of clusters of metal-poor stars, some
clusters of old but metal-rich stars and now, seen for the first time, a
population of clusters with young and metal-rich stars.

These results have been fully confirmed by spectroscopic observations made
with another of the world’s giant telescopes, the 10-metre Keck on Hawaii.

“It is a great pleasure to see two projects wholly or partly funded by
Europe – VLT and Hubble – work in concert to produce such an important
scientific result”, says Piero Benvenuti, ESA Hubble Project Scientist.
“The synergy between the most advanced ground and space telescopes
continues to prove its effectiveness, paving the way to impressive new
discoveries that would not otherwise be possible.”

The discovery of young globular clusters within old galaxies is surprising
since the stars in the giant elliptical galaxies were until now believed
to have formed during a single period early in the history of the
Universe. It is now clear that some of the galaxies may be hiding their
true nature and have indeed experienced much more recent periods of major
star formation.

Notes for editors

This press release is issued in coordination between ESA and ESO.

The Hubble Space Telescope project is an international cooperation between

The team working on these HST/VLT observations is presenting the results
at the New Horizons in Globular Cluster Astronomy conference in Padua,
Italy, 24-28 June 2002.

The team consists of Markus Kissler-Patig (European Southern Observatory,
Germany), Thomas H. Puzia (University of Munich, Germany), Stephen E. Zepf
(Yale University and Michigan State University, United States), Michael
Hilker (Sternwarte University, Bonn, Germany), Dante Minitti (Catholic
University, Santiago, Chile), Paul Goudfrooij (STScI, Baltimore, United
States) and Maren Hempel (European Southern Observatory, Munich, Germany).

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