Globular Cluster Kinematics and Dark Matter Content of the Isolated Elliptical NGC 720 by Amanda M. Schembri A thesis submitted to the Department of Physics, Astronomy and Engineering Physics in conformity with the requirements for the degree of Master of Science Queen's University Kingston, Ontario, Canada January 2011 Copyright © Amanda M. Schembri, 2011 Abstract We examine the globular cluster system (GCS) of the isolated elliptical NGC 720 using the Gemini Multi-Object Spectrograph (GMOS) and have obtained spectra for 241 candidate globular clusters (GCs) extending to a galactocentric radius of 40 kpc. Of the 241 candidates, 120 are confirmed GCs, where 46 are members of the metal-poor, blue, population and 74 are members of the metal-rich, red, population. A (g-i) =0.50 colour split is used to identify the blue and red populations. We measure the full GCS to have a rotational velocity of 50 ± 7 km/s with a position ◦ ◦ angle of θ0 = 170 ± 69 . The red population has a rotational velocity of 97 ± 14 ◦ km/s with θ0 = 147 ± 18 and the blue population has Vrot = 79 ± 7 km/s with ◦ θ0 = 89 ± 18 . The full GCS has an average velocity dispersion of 168 ± 22 km/s, for the red population is 156 ± 30 km/s and for the blue population is 181 ± 33 km/s. The velocity dispersion profile for all populations is constant with increasing radius, suggesting the presence of a dark matter halo. Using a tracer mass estimator, we +0:6 12 have measured the mass out to 40 kpc as 1:8−0:1 × 10 M for a potential which traces the dark matter profile. We also estimate the M=LV = 30 − 70. This study extends our survey of GCSs to isolated environments. i Acknowledgments I would like to thank Dave Hanes for taking me on as a student, for taking me to Chile to visit Gemini South and for all his help and insight! A huge thank you goes to Terry Bridges for all the advice he has given me, for all the hours teaching me how to reduce my data and for the effort put into helping me edit this thesis. This project would not have progressed as it did without his guidance! A huge thank you goes out to all the astro grads - for all the official (and unofficial) tea times, birthday cake celebrations, grad club visits, assignment parties, real parties, "magical food adventures" and ridiculous lunch time conversations. They were a blast and, well, magical! I would like to thank my fiance, Viktor Terlaky, for making me laugh, and for dragging me away from my desk when I refused to take breaks. He, somehow, managed to keep me sane... for the most part at least! I'd like to thank my parents for encouraging me to always persevere and to put my full effort into everything I put my mind to. Finally, I would like to thank the KMS triathalon group I coached for being so enthusiastic about learning and for putting on a smile when I put them through their paces. I learnt as much from them as (I hope) they did from me. Thank you Queen's swimming pool for existing and letting me take my frustration and excess energy out on you. Thank you everyone! ii Table of Contents Abstract i Acknowledgments ii Table of Contents iii List of Tables v List of Figures vi 1 Introduction 1 1.1 Motivation . 1 1.2 Contributions . 1 1.3 Organization of Thesis . 2 Chapter 2: Background on Globular Cluster Systems . 3 2.1 General . 3 2.2 Bimodality in Globular Clusters . 4 2.3 GCS and Formation History . 6 2.4 Dynamical Tracers . 10 2.5 X-ray Observations . 12 2.6 Specific GCS Studies . 13 Chapter 3: NGC 720 . 19 3.1 Overview of Properties . 19 3.2 X-ray Observations . 23 3.3 The Isophotes of NGC 720 . 25 3.4 Previous Determinations of the Mass of NGC 720 . 26 iii Chapter 4: Data Reduction . 28 4.1 The Data . 28 4.2 The Gemini Multi-Object Spectrograph . 33 4.3 The Reduction . 33 4.4 The Spectra . 41 4.5 Astrometry . 41 4.6 Photometry . 52 4.7 Cross-Correlation . 61 4.8 Repeat Observations . 78 Chapter 5: Modeling . 82 5.1 Rotation in the GCS of NGC 720 . 82 5.2 Velocity Dispersion . 95 5.3 Tracer Mass Estimator . 100 Chapter 6: Discussion . 108 6.1 GC Rotation . 108 6.2 Velocity and Velocity Dispersion of the GCS . 112 6.3 Mass . 116 6.4 Implications . 119 Chapter 7: Summary and Conclusions . 122 7.1 Summary . 122 7.2 Future Work . 123 7.3 Conclusion . 124 Appendix A: Tables of Data . 134 iv List of Tables 3.1 Properties of NGC 720 . 20 4.1 Location of the fields used for spectroscopy of NGC 720 . 31 4.2 Astrometry matches between fields . 79 4.3 Velocity matches between fields . 80 5.1 Summary for Results Obtained from Wrap.f90 and LoopyWrap.f90 code 89 5.2 Summary of isotropic and anisotropic mass estimates for varying α and β parameters. We set γ = 2:82 . 105 5.3 Summary of averaged masses for varying β and constant α . 107 6.1 Summary of kinematics data from other the GCSs of elliptical galaxies. References are as follows: [1] = C^ot´e et al. [2001], [2] = Woodley et al. [2010], [3] = C^ot´e et al. [2003], [4] = Bridges et al. [2006], [6] = Hwang et al. [2008], [5] = Schuberth et al. [2010]. 111 6.2 Average velocities corresponding to azimuthal angles and Figure 6.1 . 113 6.3 Summary of past estimates of the mass and M/L of NGC 720. 118 A.1 Photometry Results: In the Comments column, R and C indicate whether an object is rejected (R) from candidacy or is confirmed (C) as a GC. IP and GP indicate that there were problems obtaining photometry for either the i-band or g-band image. 134 A.2 Velocity Results: The Quality column comments on the quality of the cross-correlation fit where G = good quality, P = poor, Pf = initially poor but has been fixed. The Comments column states whether a spectra is a confirmed (C) GC or if it was rejected (R). 140 A.3 Astrometry Results: The i-band and g-band astrometry is included even though all the analysis was performed using the g-band data. The comments column specifies whether an object was confirmed (C) as a GC or rejected (R). 146 v List of Figures 2.1 Colour magnitude diagram on the left and corresponding colour dis- tribution for different magnitude ranges on the right for the M 87 GCS. There is a clear distinction between the blue and red population [Harris , 2009]. 5 4.1 The five fields for NGC 720. Field 1 is the center green box, Field 2 is the green box to the NW, Field 3 is the blue box to the SE, Field 4 is red box to the NE and Field 5 is the red box to the SW. The RA coordinate increases to the left. 32 4.2 A sample arc calibration where the initial 8 emission lines are identi- fied. These lines are identified by the user and then the gswavelength task uses these to autoidentify the rest. The x-axis is the wavelength in angstroms and the y-axis is in counts. 38 4.3 Examples of confirmed GCs with low S/N. The spectrum on the left has a S/N = 2.9 and on the right, S/N =3.5. 42 4.4 Examples of confirmed GCs with medium S/N. The spectrum on the left has a S/N = 13.0, and on the right, S/N = 11.8. In both figures, Hβ is visible at 486.1 nm and the Mg B lines are distinctly visible at 517.2 nm and 518.3 nm only for the figure on the right. 42 4.5 Examples of confirmed GCs with high S/N. The spectrum on the left has a S/N = 15.5, and on the right, S/N = 28.8. Hβ is visible at 486.1 nm and the Mg B lines are visible at 517.2 nm and 518.3 nm. 43 4.6 Examples of unusual spectra. The spectrum on the left has S/N = 14.1, and on the right, S/N =64.5. 43 4.7 Spectrum of the center of NGC 720. 44 4.8 Field 1 Image in g-band. 18 matches were found. Blue circles mark candidate objects. Green circles mark matches between the USNO B1.0 catalogue and the image. Red circles are acquisition stars used for mask centering. 46 vi 4.9 Field 2 Image in g-band. 18 matches were found. Blue circles mark candidate objects. Green circles mark matches between the USNO B1.0 catalogue and the image. Red circles are acquisition stars used for mask centering. 47 4.10 Field 3 Image in g-band. 22 matches were found. Blue circles mark candidate objects. Green circles mark matches between the USNO B1.0 catalogue and the image. Red circles are acquisition stars used for mask centering. 48 4.11 Field 4 Image in g-band. 15 matches were found. Blue circles mark candidate objects. Green circles mark matches between the USNO B1.0 catalogue and the image. Red circles are acquisition stars used for mask centering. 49 4.12 Field 5 Image in g-band. 15 matches were found.