Binary Stars in Dwarf Spheroidal Galaxies
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Binary Stars in Dwarf Spheroidal Galaxies by Meghin Elise Spencer A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Astronomy and Astrophysics) in The University of Michigan 2017 Doctoral Committee: Professor Mario L. Mateo, Chair Professor Eric F. Bell Professor Dragan Huterer Lecturer Sarah R. Loebman Professor Monica Valluri c Meghin E. Spencer 2017 All Rights Reserved [email protected] ORCID ID:0000-0003-1240-1939 ACKNOWLEDGMENTS As I reflect on the last five years of my time in grad school, I am reminded now more than ever that it takes a village to raise a grad student. Countless people have helped make this journey possible, and I would like to highlight some of the ones who played a particularly influential role in that process. First and foremost, I want to thank my research advisor, Mario Mateo. I arrived in Michigan without a plan of who I would work with or what I would research. Mario filled that gap perfectly, and reiterated time and time again that the research you do depends on where the data take you. (Although, it did take a solid three years before we finally figured out that the data really wanted us to research binary stars.) Throughout my five years at UMich, Mario gave me both the space to explore my own research questions and the guidance necessary to keep me from going astray. Despite the fact that my dissertation required no further observations, he graciously allowed me to go on 11 observing runs during which I was crowned as a jefe for his spectrograph M2FS. This also afforded me the opportunity to gain invaluable experience at a telescope and make connections with astronomers from all around the world. Above all else, I am most grateful for his receptiveness to my left field request to simultaneously change my dissertation topic and complete my final two years of school remotely. He had faith in me to remain diligent to my research outside of an academic setting and the patience to deal with time zones and technical difficulties over Skype. It has been fun playing softball (something at which he will always be better) and plugging plates (something at which I am arguably faster) ii alongside Mario. I have learned many life lessons from him and feel confident that I can use these skills in my future endeavors. I am also very grateful to the four other members of my thesis committee, who also stuck with me when I changed my dissertation topic. Their comments and criticisms have greatly improved this work. They have been a great influence to me in other realms of grad school. I have fond memories of building galaxies out of M&M's and sprinkles on my first day of grad school in Eric's class, and I really enjoyed teaching astronomy summer camp side by side with Monica and Dragan. I extend a special thank you to Sarah who helped me publish my very first paper|a project which receives a mere sentence of recognition in this dissertation|and for mentoring me through my entire graduate school career. She was a great audience for listening to many of my practice talks and provided the computing power necessary to carry out my simulations. I will miss eating chocolate while sitting on the chaise lounge in her office and discussing everything from science and teaching to children's songs and cartoon shows. I am thankful to my fellow grad students at Michigan and colleagues elsewhere in the world who have supported me along this journey. I had a great time kayaking down the Huron River, birding the arboretum, camping in the Upper Peninsula, sledding at the golf course, and playing intramural battleship in Ann Arbor with many of these grad students. Specific thanks must be given to Jeb Bailey, who was the best academic brother I could have asked for; to Marina Kounkel, who partook in many thought-provoking conversations with me while commuting on Bus 36 and over Skype; and to Hui Li who was a fantastic office mate during my second and final year of having a real office. A number of colleagues have also helped me on this journey. I thank Jon Miller for reiterating the benefits of pursuing a PhD; Matt Walker for patiently answering all of my emails, even the ones that required hours of work on his end; and Ed Olszewski for livening up many of my observing runs with unforgettable iii stories, and for enduring my late-night craziness. This research would not have been possible without the encouragement from my lifelong best friends: the Buddy Ol' Pals. My highest gratitude is extended to Stephanie Brereton for encouraging me to stick with it and navigate my own path toward my career goals; to Kasey Eck for reminding me that I wrote 200+ pages on dwarf butts (more scientifically known as dwarf galaxy posterior probability distri- butions); to Aubrey Steingraber for informing me that no dissertation is a success without using the word \paucity" (look for it) and for reading a draft of this disser- tation; and to Jessica Hartman for genuinely caring about my work even if she didn't have a clue what it meant. Last but not least, I want to acknowledge the love and support offered to me by my family. To my mom, Shelli Spencer, thank you for sending me care packages and checking in with me on a regular basis to ensure that your little train stayed on the tracks. To my dad, Brian Spencer, thank you for sparking my interest in science and astronomy at such a young age. To my brother, Andrew Spencer, thank you for keeping it real. To my grandpa, Gene Spencer, I would say this: \now you finally have a Dr. Spencer in the family. Neat stuff!" And lastly I give thanks to my husband, Aaron Beckner, for always being there for me, even when he was 2,000 miles away on the other side of the country. Aaron, your unconditional love and belief in my abilities has meant the world. We both set our sights high; mine to the stars and yours to the skies. iv TABLE OF CONTENTS ACKNOWLEDGMENTS ::::::::::::::::::::::::::: ii LIST OF FIGURES ::::::::::::::::::::::::::::::: viii LIST OF TABLES :::::::::::::::::::::::::::::::: xi LIST OF APPENDICES :::::::::::::::::::::::::::: xii ABSTRACT ::::::::::::::::::::::::::::::::::: xiii CHAPTER I. Introduction ..............................1 1.1 Denizens of the Outer Galactic Halo . .1 1.1.1 Star Formation . .2 1.1.2 Galaxy Formation . .2 1.1.3 Dwarf Galaxies and Dark Matter . .4 1.2 Binary Stars in Dwarf Galaxies . .6 1.2.1 Binaries in Ultra-Faints . .8 1.2.2 The Radial Velocity Equation . .9 1.2.3 Methods for Determining Binary Fraction . 11 1.3 Dissertation Objectives and Structure . 15 II. A Multi-Epoch Kinematic Study of Leo II ........... 20 2.1 Introduction . 20 2.2 Observations and Data Processing . 22 2.2.1 Photometry . 22 2.2.2 Spectroscopy . 25 2.2.3 Velocity Measures . 26 2.2.4 Quality Control . 30 2.3 Kinematic and Chemical Analysis . 32 2.3.1 Defining Membership . 32 v 2.3.2 Kinematic Features . 36 2.3.3 Chemical Features . 42 2.4 Summary and Conclusions . 50 III. The Binary Fraction in Leo II ................... 52 3.1 Introduction . 52 3.2 Radial Velocities . 53 3.2.1 Velocity variability . 58 3.3 Methodology . 61 3.3.1 Binary Orbital Parameters . 63 3.3.2 Method for Determining Binary Fraction . 68 3.3.3 Likelihood . 70 3.3.4 Characterizing the PPDs . 73 3.4 Binary Fraction in Leo II . 74 3.4.1 Other Considerations: Heterogeneity . 79 3.4.2 Other Considerations: Velocity Errors . 80 3.4.3 Consequences for Ultra-faints . 82 3.5 Summary and Conclusions . 83 IV. The Binary Fractions in Draco and Ursa Minor ........ 87 4.1 Introduction . 87 4.2 Velocity Data . 89 4.2.1 Data Sets . 90 4.2.2 Sample Definition . 92 4.2.3 Correcting Systematic Offsets . 97 4.2.4 Velocity Uncertainty . 98 4.2.5 Summary of Velocity Data . 102 4.3 Methodology . 103 4.3.1 Binary Parameters . 106 4.3.2 Monte Carlo Simulations . 112 4.3.3 Bayesian Technique . 114 4.3.4 Repeatability . 118 4.4 Results . 118 4.4.1 Binary Fractions Among Dwarfs . 125 4.5 Conclusions . 138 V. Summary and Conclusions ..................... 142 5.1 Velocities are a Multi-Purpose Tool . 142 5.2 Binary Fractions in Classical dSphs . 144 5.3 The Effect of Binaries in Ultra-faints . 148 5.4 Future Work . 151 5.4.1 The Value of Additional Kinematic Data . 151 vi 5.4.2 Improving the Error Model . 151 5.4.3 Determining the Period Distribution . 153 5.4.4 Alternative Definitions of β .............. 153 5.4.5 Moving Forward: More Data is the Key . 154 APPENDICES :::::::::::::::::::::::::::::::::: 156 BIBLIOGRAPHY :::::::::::::::::::::::::::::::: 285 vii LIST OF FIGURES Figure 1.1 Radial velocities caused by binaries . .7 1.2 Geometry of a binary orbit . 18 1.3 Sample velocity curves . 19 2.1 Image of Leo II spectroscopic targets on the sky . 23 2.2 Leo II color-magnitude diagram . 24 2.3 fxcor method of extracting velocities from spectra . 28 2.4 Comparison of velocity extraction techniques. 29 2.5 Velocity quality criteria from Bayesian technique . 31 2.6 Velocity histogram for Leo II . 33 2.7 Metallicity histogram of Leo II . 35 2.8 Velocity dispersion profiles of Leo II . 38 2.9 Rotation signature of Leo II . 39 2.10 Velocity vs. radius for Leo II . 41 2.11 Metallicity comparison between other published works .