Studies on the Formation, Evolution, and Destruction of Massive Star Clusters Cover: Photograph taken at White Sands, New Mexico. Courtesy of Matt Robertson. Printed by MultiCopy, Utrecht ISBN 90–393–0502–1 Studies on the Formation, Evolution, and Destruction of Massive Star Clusters Studie van de Vorming, Evolutie, en Destructie van Massieve Sterclusters (met een samenvatting in het Nederlands) Proefschrift ter verkrijging van de graad van doctor aan de Universiteit Utrecht op gezag van de Rector Magnificus, Prof. Dr. W.H. Gispen, ingevolge het besluit van het College voor Promoties in het openbaar te verdedigen op donderdag 21 april 2005 des middags te 12.45 uur door Nathan John Bastian geboren op 22 March 1978, te Milwaukee Promoter: prof. dr Henny J.G.L.M. Lamers Sterrenkundig Instituut, Universiteit Utrecht Co-Promoter: dr Markus Kissler-Patig European Southern Observatory This research has been supported in part by the Nether- lands Organization for Scientific Research (NWO). Additional support has come from the European South- ern Observatory (ESO). Contents 1 Introduction 1 1.1 Introduction to Extragalactic Star Clusters . 1 1.1.1 The Milky Way Bias . 1 1.1.2 The copious environments of young star clusters . 2 1.2 This Thesis . 5 1.2.1 Cluster Populations . 5 1.2.2 Dynamical Studies of the Massive Star Cluster W3 in the Merger Remnant NGC 7252 . 10 1.2.3 Cluster Complexes . 10 1.2.4 Future Plans . 13 Part A: The Cluster Population of M 51 15 2 Clusters in the Inner Spiral Arms of M 51: The cluster IMF and the formation history 17 2.1 Introduction . 18 2.2 Observations and reduction . 19 2.3 Selection of the clusters . 20 2.3.1 The Hα and [OIII] magnitudes . 23 2.4 Cluster evolution models . 25 2.4.1 Starburst99 cluster evolution models . 25 2.4.2 Starburst99 cluster evolution models . 25 2.4.3 The Frascati models . 28 2.4.4 The extinction curve . 30 2.5 Fitting the observed energy distributions to observed cluster models . 30 2.5.1 The fitting procedure . 30 2.5.2 Three dimensional maximum likelihood method . 31 2.5.3 Two dimensional maximum likelihood method . 33 2.5.4 Starburst99 or Frascati models? . 34 2.5.5 Uncertainties in the derived parameters . 34 2.5.6 Contamination of the cluster sample by stars? . 35 2.6 The mass versus age distribution . 35 2.7 The cluster Initial Mass Function . 37 2.8 The cluster formation history . 40 CONTENTS CONTENTS 2.9 Summary and conclusions . 42 3 The Star Cluster Population of M 51: II. Age distribu- tion and relations among the derived parameters 45 3.1 Introduction . 46 3.2 Observations, reduction and photometry . 47 3.2.1 WFPC2 Observations . 48 3.2.2 NICMOS Observations . 48 3.2.3 Source selection . 51 3.2.4 Photometry . 51 3.2.5 Incompleteness due to detection limits . 52 3.3 The selection of the clusters and the photometry master list. 54 3.3.1 Contamination of the sample by stars . 55 3.4 Determination of the cluster parameters from their energy distribution . 56 3.4.1 Adopted cluster models . 57 3.4.2 The fitting of the spectral energy distributions . 57 3.4.3 Test of the accuracy of the method . 58 3.4.4 Reduction of the sample based on energy distributions . 61 3.4.5 The elimination of stars on the basis of their energy distribution . 62 3.4.6 The effect of metallicity . 63 3.4.7 Recovery rates . 65 3.5 Extinction . 65 3.6 The age distribution of the clusters . 66 3.6.1 The mass versus age diagram . 67 3.6.2 The cluster formation history . 72 3.6.3 Spatial Dependence of the Cluster Formation Rate . 75 3.7 A population of short-lived clusters . 76 3.8 Cluster sizes . 77 3.9 Correlations with the cluster radius . 80 3.10 Conclusions . 83 Part B: The Cluster Population of M 82 87 4 Star Cluster Formation and Disruption Time-Scales: II. Evolution of the star cluster system in the fossil star- burst of M 82 89 4.1 Introduction . 90 4.1.1 Multiple starbursts in M82 . 90 4.1.2 Cluster disruption time-scales . 91 4.2 Observations . 92 CONTENTS CONTENTS 4.3 Observed cluster mass and age distributions . 93 4.4 The derived cluster formation history . 98 4.4.1 Cluster age and mass distributions . 98 4.4.2 Cluster disruption and the cluster formation history . 100 4.5 The mass distribution of clusters formed in the burst. 102 4.6 Discussion . 106 4.6.1 The cluster disruption time-scale . 106 4.6.2 The cluster formation rate . 108 4.7 Summary and Conclusions . 109 5 The Missing Link in Star Cluster Evolution 111 5.1 The Cluster Luminosity Function . 112 5.2 The intermediate-age star cluster system in M82 . 113 5.3 Detection of a turn-over in an intermediate-age CLF . 115 5.4 Summary and Implications . 117 Part C: Star Clusters in Tidal Tails 119 6 The Star Cluster Population in the Tidal Tails of NGC 6872 121 6.1 Introduction . 122 6.2 Observations, reduction and photometry . 123 6.3 The cluster population . 124 6.3.1 Colours . 124 6.3.2 Ages . 126 6.3.3 Masses . 128 6.4 The Youngest Clusters . 132 6.5 Specific U-band Luminosity . 132 6.6 A Scenario for the Formation and Evolution of Star Clusters in Tidal Tails . 135 6.6.1 Cluster formation . 135 6.6.2 Fate of the Tidal Material . 136 6.7 Conclusions . 137 Part D: Dynamical Studies of Massive Star Clusters 141 7 The dynamical mass of the young cluster W3 in NGC 7252: Heavy-Weight globular cluster or ultra compact dwarf galaxy ? 143 7.1 Introduction . 144 CONTENTS CONTENTS 7.2 Determination of velocity dispersion . 145 7.3 Structure and radius of W3 . 148 7.4 Dynamical mass of W3 . 150 7.5 Discussion . 150 7.5.1 Mass . 150 7.5.2 Clues from the κ space . 153 Part E: Star Cluster Complexes 155 8 VIMOS Spectroscopy of Young Cluster Complexes in the Antennae Galaxies 157 8.1 Introduction . 158 8.2 Observations . 159 8.2.1 VLT - VIMOS . 159 8.2.2 HST - WFPC2 . 160 8.3 Observed spectral properties . 162 8.3.1 Introduction to the spectra . 162 8.3.2 Nebular emission line features . 162 8.4 Extinction and metallicity of the complexes . 162 8.5 Ages of the cluster complexes . 165 8.5.1 Ages of the individual clusters based on photometry . 165 8.5.2 Age dating of the complexes based on their Wolf-Rayet features . 169 8.6 Interstellar matter in the cluster complexes . 171 8.6.1 Expansion velocities of the complexes and star formation rates . 171 8.6.2 Expanding shells . 172 8.7 Formation of the complexes . 172 8.7.1 Relation to giant molecular clouds . 174 8.8 Stability of the complexes . 175 8.9 Conclusions . 175 9 Hierarchical Star Formation in M51: Star/Cluster Complexes 181 9.1 Introduction . 182 9.2 Observations . 183 9.2.1 HST-WFPC2 observations . 183 9.2.2 Bima CO observations . 184 9.3 Properties of the complexes . 185 9.3.1 Sizes of the complexes . ..