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Globular Clusters As Probes of Galaxy Formation

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Globular Clusters As Probes of Galaxy Formation Durham E-Theses Globular clusters as probes of galaxy formation Beasley, Michael Andrew How to cite: Beasley, Michael Andrew (2001) Globular clusters as probes of galaxy formation, Durham theses, Durham University. Available at Durham E-Theses Online: http://etheses.dur.ac.uk/4949/ Use policy The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in Durham E-Theses • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders. Please consult the full Durham E-Theses policy for further details. Academic Support Oce, Durham University, University Oce, Old Elvet, Durham DH1 3HP e-mail: [email protected] Tel: +44 0191 334 6107 http://etheses.dur.ac.uk Globular Clusters as Probes of Galaxy Formation Michael Andrew Beasley A thesis submitted to the University of Durham in accordance with the regulations for admittance to the Degree of Doctor of Philosophy. The copyright of this thesis rests with the author. No quotation from it should be published without his prior written consent and information derived from it should be acknowledged. Department of Physics University of Durham January 19, 2001 The copyright of this thesis rests >vith the author. No quotation from it should be |)uhlishcd in any form, including Electronic and the Internet, without the author's prior written consent. All information derived from this thesis must he acknowledged appropriately. '...as observations have accumulated, the subject has become, to my mind at least, more mysterious and more inapproachable.' William Parsons, Third Earl ofRosse Abstract Globular Clusters as Probes of Galaxy Formation by Michael Andrew Beasley Observations and analysis of globular cluster systems associated with three galaxy types are presented. Spectroscopy of globular cluster (GC) candidates in the Sculptor spirals NGC 253 and NGC 55 has identified 15 GCs in these galaxies. This spectroscopic sample, combined with plate scans, in• dicates total GC populations consistent with that expected for their luminosity and morphological type. From these data, we define new GC samples for spectroscopy. Radial velocities of 87 GCs in the Virgo elliptical NGC 4472 have been obtained, yielding data for 144 GCs when combined with previous studies. We find the blue GCs have significantly higher velocity dispersion than the red GCs, with little rotation in either population. The GCs dispersion profile declines slowly, yielding mass profiles consistent with X-ray data. We find a steeply rising MjL ratio, indicative of a massive dark halo surrounding this galaxy. From line-strengths of the GCs, we derive ages and metallicities for the GCs using simple stellar population (SSP) models. We find that the GCs are old and coeval and the bimodality seen in their colours reflects metallicity rather than age differences. The GCs exhibit solar abundance ratios, and both subpopulations show evidence for radial metallicity gradients. We have obtained high S/N spectra for 64 star clusters in the Large Magellanic Cloud. We measure their Lick indices to test the age and metallicity calibration of SSP models by comparison with literature values. We find our metallicities are consistent, although the values from our integrated spectra are slightly higher. The agreement of the ages for the old GCs is good, but is somewhat poorer for the youngest clusters. We obtain an age-metallicity relation for the clusters consistent with the galaxy's field stars. We show first results of a project to investigate the age and metallicity distributions of globular cluster systems using semi-analytic models of galaxy formation. IV Declaration The research undertaken in this thesis was performed during the period 1997-2000 whilst the author was a research student under the supervision of Dr Ray Sharpies in the Department of Physics at the University of Durham. This work has not been submitted for any other degree at the University of Durham or at any other University. The work shown in this thesis is entirely my own with the following exceptions : Chapter 2.0 was undertaken with Dr. Ray Sharpies. Chapter 3 represents the results of a project initiated in 1996 in collaboration with Ray Sharpies, Terry Bridges, Steve Zepf, Doug Geisler, Keith Ashman and Dave Hanes. The semi-analytic results in Chapter 5 are very much an ongoing project, and would not have been possible without the work of Carlton Baugh. Much of the material in Chapters 2 and 3 has appeared in the following publications: • Beasley M.A., Sharpies R.M., 2000, MNRAS, 331, 673 • Beasley M.A., Sharpies R.M., Bridges T.J., Hanes, D.A., Zepf, S.E., Ashman, K.M., 2000, MNRAS, 318, 1249 • Stephen E. Zepf, Michael A. Beasley, Terry J. Bridges, David A. Hanes, Ray M. Sharpies, Keith M. Ashman, Doug Geisler, 2000 AJ, in press The copyright of this thesis rests with the author. No quotation from it should be published without his prior written consent and information derived from it should be acknowledged. To my Mum and Dad VI Acknowledgements The PhD process is not simply an academic work, but rather like being set adrift on a raft in the ocean. Without the tremendous support of mermen and mermaids (well, friends, family and colleagues), I'd be amazed if anyone ever finished, er ever. The person I have to thank most is Ray Sharpies, my supervisor. If he hadn't told me to stop being silly the night before I went out for my first observing run in Hawaii, I wouldn't have made it to my second year. I am also indebted to two people at LJMU without whom I'd never have started a PhD, Phil James and Tim O'Brien. On the science front, I must also thank my collaborators, Terry Bridges, Dave Hanes (I remem• ber the jelly worms), Doug Geisler, Keith Ashman and Steve Zepf. Also thanks to Harald for his vast knowledge of spectroscopic things, Alex for his endless enthusiasm, and to Carlton who works very hard. Hi and thanks to the people who have gone on to do even bigger and better stuff; Fiona-I miss your joi de vive, Andrew (well I always new you'd do well), Scott Kay (printer repair expertise), SJH (for his dodgy jacket), Steve Burby, DJ Simon Shaw, and Seb. Thanks to people in the department who provide entertainment; Geoff, Rich, Dave, Kev, Dijana, Infra-Red Peder Norberg, Carlos (nice mole), Graham x 2, Mike^, Joy, Sam, Harald, Chris, Pat and Fraser. To Dan & Ade I say 'pies' and thanks for the moral support. To the mountain bikers Davey, Michael, Ian...., why did / always eat the mud?? James and Louisa deserve special mention; James for his fantastic curries and punctuality, Louisa for broadening certain horizons. There's not enough space here to thank my Mum and Dad properly, so I'll just say 'thanks'. Mum, keep acting, politicking and smiling, Dad - keep flying those planes! Also, a BIG thanks to my long-suffering brothers Gareth and David and also to Jitka for putting up with the rest of the family. Oh, and Amaya, mas caramelos porfavor. Contents Chapter 1 Introduction 1 1.1 Preamble 1 1.2 What is a Globular Cluster? 3 1.2.1 Physical Characteristics 3 1.2.2 Metallicities of Globular Clusters . 4 1.2.3 The Hertzsprung-Russell Diagram 5 1.3 Globular Cluster Systems 8 1.3.1 Historical Background 8 1.3.2 Colour Distributions . 10 1.3.3 Metallicity Distributions 11 1.3.4 Young Globular Clusters 13 1.4 Correlations with the Host Galaxy 14 1.4.1 Specific Frequency 14 1.4.2 A Metallicity-Luminosity Relation? 16 1.5 Outline of this Thesis 17 Chapter 2 Globular Clusters in the Sculptor Group 19 2.1 Introduction 19 2.1.1 The Sculptor Group 21 2.2 Spectroscopy of Globular Clusters in NGC 55 and NGC 253 24 2.2.1 Selection of the Sample . 24 2.2.2 Observations 24 2.2.3 Data Reduction 25 Vlll 2.3 Velocities of the Spectroscopic Sample 26 2.3.1 Radial Velocities 26 2.3.2 Identification of Globular Clusters 31 2.4 Properties of the Globular Cluster Systems 35 2.4.1 Velocity and Spatial Distribution 35 2.4.2 Metallicities of the Globular Clusters 37 2.5 Definition of a New Sample of Globular Clusters 41 2.5.1 COSMOS Plate Scans 41 2.5.2 A New Cluster Sample 43 2.6 Correlations Between Spiral Galaxies and their GCS 53 2.6.1 Specific Frequencies 53 2.6.2 Mean Metallicities 55 2.7 Summary and Conclusions 58 Chapter 3 Spectroscopy of Globular Clusters in M 49 59 3.1 Introduction 59 3.1.1 Formation Models for Globular Cluster Systems 61 3.1.2 Spectroscopy of Globular Clusters in Giant Ellipticals . 63 3.1.3 NGC 4472 64 3.2 Spectroscopy of the NGC 4472 Globular Clusters 67 3.2.1 Observations 67 3.2.2 Data Reduction 70 3.2.3 Radial Velocities 72 3.3 Line-Strengths of the Globular Clusters 79 3.3.1 Co-adding the Spectra 79 IX 3.3.2 Measuring the Line-Strength Indices 82 3.3.3 Uncertainties in the Indices 87 3.3.4 The Measured Indices 90 3.4 Metallicities and Ages 92 3.4.1 Fiducial Globular Clusters 92 3.4.2 The Worthey Models 94 3.4.3 Metallicities of the Globular Clusters 98 3.4.4 Radial Gradients in the Globular Cluster System 101 3.4.5 Cluster Ages 104 3.5 Kinematics of the Cluster System 112 3.5.1 Velocity Dispersions 112 3.5.2 Rotation in the Cluster System 116 3.5.3 Mass of NGC 4472 119 3.6 Discussion 119 3.6.1 Results of the Line-Strength Analysis 119 3.6.2 Results of the Kinematical Analysis 121 3.6.3 Formation of the NGC 4472 GCS 122 3.7 Summary and Conclusions 123 Chapter 4 Star Clusters in the LMC 125 4.1 Introduction 125 4.1.1 The Large Cloud 126 4.1.2 A Potted History of the Large Cloud Clusters 127 4.1.3 Integrated Techniques 129 4.1.4 Individual Stellar Studies 131 X 4.2 Obtaining Spectra of the Star Clusters 133 4.2.1 Sample Selection for Spectroscopy 133 4.2.2 The FLAIR System 135 4.2.3 Estimating the Sky Contribution 139 4.2.4 Observations 140 4.3 Standard Data Reduction 149 4.3.1 The Overscan Correction 149 4.3.2 Bias Frames 149 4.3.3 Flat Fields 149 4.3.4 Arc Frames 150 4.3.5 Preparing the Science Frames 150 4.3.6 Tracing and Extraction of the Spectra 151 4.3.7 Wavelength Calibration 152 4.3.8 Sky Subtraction .
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