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The Galactic open cluster Westerlund 1 was found only a few years ago to bemuch more massive than previously thought, with evidence suggesting its mass tobe in excess of ~10^5 Msun, in the range spanned by young extragalactic starclusters. Unlike those clusters its proximity makes spatially resolved studiesof its stellar population feasible. It is therefore the ultimate template for ayoung, massive star cluster, permitting direct comparison of its propertieswith measurements of velocity dispersion and dynamical mass for spatiallyunresolved extragalactic clusters.  To this end, we used the long slit near-infrared spectrograph VLT/ISAAC toobserve the CO bandhead region near 2.29 micron scanning the slit across thecluster centre during the integration. Spatially collapsing the spectra alongthe slit results in a single co-added spectrum of the cluster, comparable towhat one would obtain in the extragalactic cluster context.  This spectrum was analysed the same way as the spectra of almost point-likeextragalactic clusters, using red superiant cluster members as velocitytemplates.  We detected four red supergiants which are included in the integratedspectrum, and our measured velocity dispersion is 5.8 km/s. Together with thecluster size of 0.86 pc, derived from archival near-infrared SOFI-NTT images,this yields a dynamical mass of 6.3x10^4 Msun. While this value is not to beconsidered the final word, there is at least so far no sign for rapid expansionor collapse.Comment: 6 pages, 5 figures, A&A accepte

Medium resolution 2.3 ${\mathsf \mu}$m spectroscopy of the massive Galactic open cluster Westerlund 1