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

Abstract

We present evidence that some of the compact, luminous, young star clusters discovered recently through images taken with the Hubble Space Telescope (HST) have masses comparable to those of old Galactic globular clusters. The super star in the center of the nearby amorphous galaxy NGC 1705 has been observed with high dispersion at optical wavelengths using the HIRES echelle spectro-graph on the Keck 10 m telescope. Numerous weak metal lines arising from the atmospheres of cool supergiants have been detected in the integrated spectrum, permitting a direct measurement of the line-of-sight stellar velocity dispersion through cross-correlation with a template star; the result is σ* 11.4 ± 1.5 km s–1. Assuming that the system is gravitationally bound and using a cluster size measured from HST images, we apply the virial theorem to obtain the dynamical mass. Its derived mass [(8.2 ± 2.1) × 104 M☉], mass density (2.7 ± 104 pc–3), and predicted mass-to-light ratio after aging by 10-15 Gyr [0.6-1.6(M/LV)☉] 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 of the bright clusters in NGC 1569, which was discussed earlier this year by Ho & Filippenko. We also observed the brightest cluster in the Magellanic irregular galaxy NGC 4214, but no lines suitable for measuring its velocity dispersion were detected, most likely because of the very young age of the cluster. Although these observations need to be extended to a much larger sample of objects before generalizations can be made concerning the nature of similar clusters observed in other galaxies, our preliminary results are tantalizing and strongly suggest that, at least in two cases, we are witnessing globular clusters in their extreme youth.