Young star cluster complexes in NGC 4038/39

Integral field spectroscopy using VIMOS-VLT

¹ Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands e-mail: bastian@star.ucl.ac.uk
² European Southern Observatory, Karl-Schwarzchild-Strasse 2, 85748 Garching b. München, Germany
³ CRAL–Observatoire de Lyon, 9 avenue Charles André, 69561 Saint-Genis-Laval Cedex, France
⁴ University of Oxford, Denys Wilkinson Building, Keble Road, Oxford, OX13RH, UK

Received: 7 July 2005
Accepted: 8 September 2005

Abstract

We present the first results of a survey to obtain Integral Field Spectroscopy of merging galaxies along the Toomre Sequence. In the present work, we concentrate on the star cluster complexes in the Antennae galaxies (NGC 4038/39) in the overlap region as well as the nuclear region of NGC 4038. Using optical spectroscopy we derive the extinction, age, metallicity, velocity, velocity dispersion of the gas and star formation rate for each of the eight complexes detected. We supplement this study with archival HST-WFPC2 U, B, V, Hα, and I band imaging. Correcting the observed colours of the star clusters within the complexes for extinction, measured through our optical spectra, we compare the clusters with simple stellar population models, with which we find an excellent agreement, and hence proceed to derive the ages and masses of the clusters from comparison with the models. In five of the complexes we detect strong Wolf-Rayet emission features, indicating young ages (3–5 Myr). The ionized gas surrounding the complexes is expanding at speeds of 20-40 km s^-1. This slow expansion can be understood as a bubble, caused by the stellar winds and supernovae within the complexes, expanding into the remnant of the progenitor giant molecular cloud. We also find that the complexes themselves are grouped, at about the largest scale of which young star clusters are correlated, representing the largest coherent star forming region. We show that the area normalized star formation rates of these complexes clearly place them in the regime of star forming regions in starburst galaxies, thereby justifying the label of localized starbursts. Finally, we estimate the stability of the complexes, and find that they will probably loose a large fraction of their mass to the surrounding environment, although the central regions may merge into a single large star cluster.