High‐Dispersion Spectroscopy of a Luminous, Young Star Cluster in NGC 1705: Further Evidence for Present‐Day Formation of Globular Clusters

We present evidence that some of the compact, luminous, young star clustersrecently discovered through images taken with the Hubble Space Telescope (HST)have masses comparable to those of old Galactic globular clusters. The "superstar cluster" in the center of the nearby amorphous galaxy NGC 1705 has beenobserved with high dispersion at optical wavelengths using the HIRES echellespectrograph on the Keck 10 m telescope. Numerous weak metal lines arising fromthe atmospheres of cool supergiants have been detected in the integratedspectrum, permitting a direct measurement of the line-of-sight stellar velocitydispersion through cross-correlation with a template star; the result is$\sigma_*$ = 11.4$\pm$1.5 \kms. Assuming that the system is gravitationallybound and using a cluster size measured from HST images, we apply the virialtheorem to obtain the dynamical mass. Its derived mass [(8.2$\pm$2.1) x 10$^4$$M_{\odot}$], mass density (2.7 x 10$^4$ $M_{\odot}$ pc$^{-3}$), and predictedmass-to-light ratio after aging by 10--15 Gyr [0.6--1.6 ($M/L_V$)$_{\odot}$]closely resemble those of the majority of evolved Galactic globular clusters.The central cluster in NGC 1705 appears to be very similar in nature to one ofthe bright clusters in NGC 1569, which is discussed by Ho \& Filippenko (1996).We also observed the brightest cluster in the Magellanic irregular galaxy NGC4214, but no lines suitable for measuring its velocity dispersion weredetected, most likely because of the very young age of the cluster. Althoughthese observations need to be extended to a much larger sample of objectsbefore generalizations can be made concerning the nature of similar clustersobserved in other galaxies, our preliminary results are tantalizing andstrongly suggest that, at least in two cases, we are witnessing globularComment: To appear in the Astrophysical Journal. LaTex, pages including 8 postscript figures. AAStex macros include