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Globular star clusters have a remarkable characteristic: the typical
number of stars they contain appears to be about the same throughout
the Universe. This is in contrast to much younger stellar clusters,
which can contain almost any number of stars, from fewer than 100 to
many thousands. The team of scientists proposes that this difference
can be explained by the conditions under which globular clusters
formed early on in the evolution of their host galaxies.
The researchers ran simulations of isolated and colliding galaxies, in
which they included a model for the formation and destruction of
stellar clusters. When galaxies collide, they often generate
spectacular bursts of star formation (“starbursts”) and a wealth of
bright, young stellar clusters of many different sizes. As a result it
was always thought that the total number of star clusters increases
during starbursts. But the Dutch-German team found the opposite result
in their simulations.
While the very brightest and largest clusters were indeed capable of
surviving the galaxy collision due to their own gravitational
attraction, the numerous smaller clusters were effectively destroyed
by the rapidly changing gravitational forces that typically occur
during starbursts due to the movement of gas, dust and
stars. The wave of starbursts came to an end after about 2 billion years
and the researchers were
surprised to see that only clusters with high numbers of stars had
survived. These clusters had all the characteristics that should be
expected for a young population of globular clusters, as they would
have looked about 11 billion years ago.
Dr Kruijssen comments: “It is ironic to see that starbursts may
produce many young stellar clusters, but at the same time also destroy
the majority of them. This occurs not only in galaxy collisions, but
should be expected in any starburst environment. In the early
Universe, starbursts were commonplace – it therefore makes perfect
sense that all globular clusters have approximately the same large
number of stars. Their smaller brothers and sisters that didn’t
contain as many stars were doomed to be destroyed.”
According to the simulations, most of the star clusters were destroyed
shortly after their formation, when the galactic environment was still
very hostile to the young clusters. After this episode ended, the
surviving globular clusters have lived quietly until the present day.
The researchers have further suggestions to test their ideas. Dr
Kruijssen continues: “In the nearby Universe, there are several
examples of galaxies that have recently undergone large bursts of star
formation. It should therefore be possible to see the rapid
destruction of small stellar clusters in action. If this is indeed
found by new observations, it will confirm our theory for the origin
of globular clusters.”
The simulations suggest that most of a globular cluster’s traits were
established when it formed. The fact that globular clusters are
comparable everywhere then indicates that the environments in which
they formed were very similar, regardless of the galaxy they currently
reside in. In that case, Dr Kruijssen believes, they can be used as
fossils to shed more light on the conditions in which the first stars
and galaxies were born.
Original publication:
Kruijssen et al,
"Formation versus destruction: the evolution of the star cluster population in galaxy mergers",
Monthly Notices of the Royal Astronomical Society, in press
 http://arxiv.org/abs/1112.1065
Science Contact:
Dr Diederik Kruijssen
Max-Planck Institute for Astrophysics
Garching, Germany
Tel: +49 (0)89 30000 2241
E-mail: kruijssen mpa-garching.mpg.de
Media Contact:
Dr Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7734 3307 x214
Mob: +44 (0)794 124 8035
Email: rmras.org.uk
Dr Hannelore Hämmerle
Press officer
Max-Planck Institute for Astrophysics
Garching, Germany
Tel: +49 (0)89 30000 3980
E-mail: prmpa-garching.mpg.de
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