The temporal and spatial evolution of the starburst in ESO 338-IG04 as probed by its star clusters^*

¹ Stockholm Observatory, 106 91 Stockholm, Sweden
² Uppsala University, Dept. of Astronomy and Space Physics, Box 515, 75120 Uppsala, Sweden

Corresponding author: G. Östlin, ostlin@astro.su.se

Received: 8 November 2002
Accepted: 20 June 2003

Abstract

ESO 338-IG04, also known as Tololo 1924-416, is a well known luminous blue compact galaxy in the local universe. Images obtained with the Hubble Space Telescope (HST) have shown that the central starburst region is composed of numerous bright point sources – young star clusters, surrounded by a population of old and intermediate age globular clusters. In this paper we use Ultra-Violet (UV) and optical HST photometry in five bands, and an extensive set of spectral evolutionary synthesis scenarios to investigate the age and masses of 124 star clusters. The very small reddening makes ESO 338-IG04 an excellent laboratory for studying the formation of such objects. We find that a careful treatment of the nebular emission component is crucial when modelling the broad-band colours of young starburst regions. We have used the star clusters to trace the temporal and spatial evolution of the starburst, and to put constraints on the star formation activity over a cosmological time-scale. The present starburst has been active for about 40 Myr and shows evidence for propagating star formation and structures triggered by galactic winds. A standard Salpeter initial mass function (IMF) extending up to 120 provides the best fit to the data, although a flatter IMF cannot be excluded. The compact star clusters provide 30-40% of the UV luminosity and star formation activity. We find no evidence for dust obscuration even among the youngest ( Myr) clusters, and we propose that this may be related to a short time-scale for destruction of dusty molecular clouds. Over a longer time-scale, we find evidence for previous cluster formation epochs – notably one a couple of Gyr ago. The fraction of the galaxy's stellar mass contained in compact star clusters is found to be several percent, which is an unusually high value. The intermediate age clusters show a flattened space distribution which agrees with the isophotal shape of the galaxy, whereas the oldest clusters seem to have a spherical distribution indicating that they formed prior to the rest of the galaxy.