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The Globular Cluster System of NGC 4365

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The Globular Cluster System of NGC 4365 The Globular Cluster System of NGC 4365 Christina Blom Presented in fulfillment of the requirements of the degree of Doctor of Philosophy May 2013 Faculty of Information and Communication Technology Swinburne University i Abstract This thesis presents a study of the globular cluster (GC) system of NGC 4365 and the GCs associated with the W 0 group of galaxies, of which NGC 4365 is the dominant galaxy. We use the analysis of these GC systems to investigate the evolutionary history of NGC 4365 as well as the ongoing interactions within the W 0 group. To analyse the photometric properties of NGC 4365's GC system we combined three filter imaging from the SuprimeCam instrument on the 8m Subaru telescope with eight, two filter, pointings from the Advanced Camera for Surveys on the Hubble Space Tele- scope. To analyse the kinematic properties of NGC 4365's GC system we obtained spectra for over 250 GCs around NGC 4365 from the DEep Imaging Multi-Object Spectrograph (DEIMOS) on the Keck II telescope. The photometric properties of the group GCs were analysed with square degree, three filter imaging from the MegaCam instrument on the Canada-France-Hawaii Telescope. We confirmed that NGC 4365 hosts three GC subpopulations, the usual blue and red GC subpopulations plus an additional subpopulation at intermediate colour: the green subpopulation. Photometric analysis showed that the three subpopulations have distinct radial and azimuthal distributions, different median sizes and mass distributions drawn from different populations. Using recession velocities calculated from GC spectra we also determined that each GC subpopulation rotates about the galaxy in a different direction. Analysis of the spatial distribution of GCs around NGC 4365 in the wider W 0 group environment uncovered a stream overdensity of GCs between NGC 4365 and a nearby small lenticular galaxy NGC 4342, extending South West beyond NGC 4342. This GC stream is spatially coincident with a stellar stream recently presented in the literature. We found that the recession velocities of the stream GC are consistent with the recession velocity measured from NGC 4342's starlight. We conclude that NGC 4365 formed two GC subpopulations during two separate in situ, dissipative star formation episodes (the green GCs forming before the red GCs) and accreted the blue GCs from smaller galaxies throughout its evolutionary history. We also conclude that NGC 4365 is currently accreting blue GCs and stars from the small lenticular galaxy NGC 4342. As predicted by the hierarchical merging models of ΛCDM, we have observed evidence of ongoing growth of the giant elliptical galaxy NGC 4365 in the nearby Universe. ii iii Acknowledgements I begin by acknowledging my parents for their love, guidance, motivation and patience. I have always known that I am loved by you and that knowledge let me be the exploring and questioning person that started this thesis. You have guided me through the maze of getting educated with my curiosity and joy of discovery intact. I am especially grateful for the constant words of encouragement to dream big, to work hard, to keep my eyes on the goal and to take one step forward after the other. Without my Ph.D. supervisor Prof. Duncan A. Forbes, I would not have had access to the data that contained the discoveries presented in this thesis. More importantly I would not have had the skills or insight to see what was to be uncovered. Thank you for your patient teaching, daily availability for questions and discussions, openness to explore new ideas and consistent reminders of short-term goals. To my co-supervisors; thank you Dr Lee R. Spitler for the recurrent question `What does it mean?' and thank you Prof. Alister Graham for encouraging me to pay attention to every detail. I am also indebted to my collaborators Prof. Jean P. Brodie, Dr Caroline Foster, Dr Aaron J. Romanowsky and Dr Jay Strader for their support and critical assessment of my work. I would also like to thank the staff and students at the Centre for Astrophysics and Supercomputing, Swinburne University for the friendship, community and advice over the years. In particular I would like to thank Dr. Lina Levin, Vincenzo Pota, Christopher Usher, Sreeja Kartha and Nicola Pastorello for your individual contributions to life in our corner of the office and my Ph.D. journey. Swinburne University has generously supported me through my Ph.D. with scholar- ships as well as an administrative backbone aiming to give students the best resources for their degree. Thank you, my beloved Michael D. Smith, my husband and best friend. You have been steady, patient and unfailingly kind and generous. I am privileged to be adventuring through this world with you and I need to tell you that I would not have made it through this challenge without you. My family and friends scattered throughout the globe have been for me `God's love with skin on' during my Ph.D. I would be lost without His Light and I feel honoured to have had the opportunity to discover parts of His Universe. iv v Declaration The work presented in this thesis has been carried out in the Centre for Astrophysics & Supercomputing at Swinburne University of Technology between 2009 and 2013. This thesis contains no material that has been accepted for the award of any other degree or diploma. To the best of my knowledge, this thesis contains no material previously published or written by another author, except where due reference is made in the text of the thesis. The content of the chapters listed below has appeared in refereed journals. Minor alterations have been made to the published papers in order to maintain argument continuity and consistency of spelling and style. • Chapter 2, Chapter 3, Appendices A and B have been published together as: “Wide-field imaging of NGC 4365's globular cluster system: the third subpopulation revisisted" Blom, C., Spitler, L. R. and Forbes, D. A., 2012, MNRAS, 420, 37 • Chapter 4 and Appendix C have been published together as: \The SLUGGS survey: globular cluster system kinematics and substructure in NGC 4365" Blom, C., Forbes, D. A., Brodie, J. P., Foster, C., Romanowsky, A. J., Spitler, L. R. and Strader, J., 2012, MNRAS, 426, 1959 • Chapter 5 has been submitted to MNRAS as: \The SLUGGS survey: New evidence for a tidal interaction between the early type galaxies NGC 4365 and NGC 4342" Blom, C., Forbes, D. A., Foster, C., Romanowsky, A. J. and Brodie, J. P., 2013 Contributions to the papers from coauthors are included in this work for clarity and continuity. All work is my own, unless a Section or piece of work is specifically attributed to a coauthor by name. Christina Blom Melbourne, Victoria, Australia 2013 vi Contents Abstract i Acknowledgements iii Declaration v List of Figures xi List of Tables xv 1 Introduction 1 1.1 Globular clusters . 2 1.2 Globular cluster systems . 2 1.2.1 Bimodality . 3 1.2.2 Trends within GC systems . 4 1.2.3 GC system trends . 5 1.3 Early-type galaxies . 6 1.4 Using globular cluster systems to study galaxy evolution . 7 1.5 Thesis outline . 8 2 Observing extragalactic globular clusters 9 2.1 Introduction . 9 2.2 Subaru/Suprime Cam Data . 11 2.2.1 Observations and data reduction . 11 2.2.2 Photometry and calibration . 11 2.3 HST/ACS Data . 13 2.3.1 Observations and object measurement . 13 2.4 GC candidate Selection . 14 2.4.1 HST/ACS GCs . 14 2.4.2 Subaru/SuprimeCam GCs . 16 2.4.3 Contamination and completeness . 19 2.4.4 Overview of GC selection . 21 3 Wide-field imaging of NGC 4365's globular cluster system 23 3.1 Introduction . 23 3.2 Analysis of the galaxy light . 25 vii viii Contents 3.2.1 Parameters of the isophotal fits . 25 3.3 Analysis of the GC System . 27 3.3.1 Characterising the total GC System . 27 3.3.2 Radial colour gradients . 34 3.3.3 Colour-magnitude trends . 36 3.3.4 Quantifying the GC subpopulations . 38 3.3.5 Characterising the GC system subpopulations . 42 3.3.6 Comparison with galaxy surface brightness . 53 3.4 Discussion . 53 3.5 Summary and Conclusions . 57 4 Globular cluster system kinematics and substructure in NGC 4365 59 4.1 Introduction . 59 4.2 Spectroscopic sample . 63 4.2.1 Data acquisition . 63 4.2.2 Obtaining line-of-sight velocities . 65 4.2.3 Low velocity GCs . 69 4.3 Kinematics of the GC subpopulations . 71 4.3.1 Kinematic model description . 71 4.3.2 Kinematics as a function of colour . 73 4.3.3 Dividing the sample into three subpopulations . 77 4.3.4 Radial kinematics for three subpopulations . 79 4.4 Discussion . 84 4.5 Conclusions . 87 5 Tidal interaction between NGC 4365 and NGC 4342 91 5.1 Introduction . 91 5.2 Photometric Analysis . 93 5.2.1 Imaging Data . 93 5.2.2 Globular cluster candidate selection . 93 5.2.3 Spatial distribution . 97 5.2.4 Colour distribution . 101 5.3 Spectroscopic Analysis . 106 5.3.1 Spectroscopic Data . 106 5.3.2 Velocity selection . 108 5.4 Kinematic properties of NGC 4342's GC system . 111 Contents ix 5.5 Dark matter constraints . 113 5.6 Discussion . 116 5.7 Conclusions . 118 6 Conclusions and future work 119 6.1 Chapter Summary . 119 6.2 Avenues for Further Research . 122 6.2.1 Substructure within galaxy groups and clusters . 122 6.2.2 Searching for the causes of significant kinematic misalignment . 123 6.3 Broad impact of this work .
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