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Extragalactic young star clusters have been observed over the last years
as attractive laboratories to study the star formation process, because
they are young (=> still hosting many massive stars), very massive
(=> no strong effect of stochastic IMF sampling) and often come in large
numbers in all kinds of environments. The disadvantage of lacking spatial
resolution had to be suffered, because no really high-mass cluster with a
few times 10^5 solar masses was known in the Galaxy. One explicit assumption
that went into the determination of dynamical masses was that of Virial equilibrium.
However, over the last ~two years, it turned out that this may be incorrect for
many clusters.
On the bright side, it turned out that a young Milky Way cluster, Westerlund 1,
is much more massive than previously thought. It is only one target, but neverless we
tried to use it as a reference object to determine the dynamical state of the
cluster, or more specifically, what dynamical mass we would derive for this
cluster if we observed it in a similar way like the other extragalactic clusters.
For this purpose, we obtained medium resolution near-infrared spectroscopy with
ISAAC and would present the results, together with the impact that a false
assumption of Virial equilibrium would have on our data (and those of others),
and if there is a way of telling with existing or future observations which
clusters are/could be affected.
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