Dynamical masses of two young globular clusters in the blue compact galaxy ESO 338–IG04

¹ Stockholm Observatory, AlbaNova University Center, Stockholms Center for Physics, Astronomy and Biotechnology, Roslagstullsbacken 21, 106 91 Stockholm, Sweden e-mail: [ostlin@;robert]@astro.su.se
² Uppsala Astronomical Observatory, Box 515, 751 20 Uppsala, Sweden

Received: 2 November 2005
Accepted: 26 September 2006

Abstract

We present high-resolution échelle spectroscopy, obtained with the UVES spectrograph on ESO/VLT, of two luminous star clusters in the metal-poor blue compact galaxy ESO 338–IG04 at a distance of 37.5 Mpc. Cross-correlating with template stars, we obtain line-of-sight velocity dispersions of 33 and 17 km s^-1. By combining with size estimates from Hubble Space Telescope images we infer dynamical masses of   and   for the two clusters, making them among the most massive known. The less massive cluster is the faintest cluster for which a dynamical mass has yet been obtained. In both clusters we detect Balmer absorption lines which we use to estimate their ages. From the younger (~6 Myr) and more massive cluster, we detect H ii λ4686 emission of intermediate width, indicating the presence of very massive O-stars. Moreover, analysis of the [O iii] λ5007 and Hα emission lines from the region near the younger cluster indicates that it is associated with a bubble expanding at ~40 km s^-1. We also see from the Na i D absorption lines indications of neutral gas flows towards the younger cluster. We compare the dynamical masses with those derived from photometry and discuss implications for the stellar initial mass function (IMF) in each cluster. Both clusters are compatible with rather normal IMFs which will favour their long-term survival and evolution into massive bona fide globular clusters.