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Thesis Derives from Three Projects Conducted Dur- Ing My Ph.D, Focusing on Both the Milky Way and the Magellanic Clouds

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Thesis Derives from Three Projects Conducted Dur- Ing My Ph.D, Focusing on Both the Milky Way and the Magellanic Clouds GALACTICSTRUCTUREWITHGIANTSTARS james richard grady Corpus Christi College University of Cambridge March 2021 This dissertation is submitted for the degree of Doctor of Philosophy James Richard Grady: Galactic Structure with Giant Stars, Corpus Christi College University of Cambridge, © March 2021 ABSTRACT The content of this thesis derives from three projects conducted dur- ing my Ph.D, focusing on both the Milky Way and the Magellanic Clouds. I deploy long period variables, especially Miras, as chronome- ters to study the evolution of Galactic structure over stellar age. I study red giants in the Magellanic Clouds, assign them photometric metallicities and map large scale trends both in their chemistry and proper motions. In Chapter 1 I provide an overview of the historical observations that underpin our current understanding of the Galactic components. Specifically, I detail those pertaining to the Galactic bulge, the Galactic disc and the Magellanic Clouds as it is these that constitute the main focus of the work in this thesis. In Chapter 2 I collate a sample of predominately oxygen-rich Mira variables and show that gradients exists in their pulsation period pro- files through the Galaxy. Under the interpretation that the period of Miras correlates inversely with their stellar age, I find age gradients consistent with the inside-out disc formation scenario. I develop such analysis further in Chapter 3: seizing on the Miras provided by Gaia DR2, I observe them to trace the Galactic bulge/bar and disc. With the novel ability to slice both components chronolog- ically at once, the old disc is seen to be stubby; radially constricted and vertically extended. The younger disc is vertically thinner and more extensive radially, providing further observational evidence for inside-out formation of the Milky Way disc. The longer period and hence younger bulge Miras show a clear bar-like morphology bear- ing the classic X-shape. The old, short period bulge Miras appear to constitute a separate population, decoupled from the inclined bar with little evidence for having buckled. Chapter 4 details a chemo-kinematic analysis of red giants residing in the Magellanic Clouds. I utilise optical and infra-red photometry to assign photometric metallicity estimates to the giants. The metal- licity structure in the larger Cloud correlates with known stellar over- densities induced from historic perturbations. I observe the smaller Cloud to be disrupting, identifying stellar debris trailing the dwarf. iii DECLARATION This thesis is the result of my own work and includes nothing which is the outcome of work done in collaboration except as declared below and specified in the text1: • Chapter 2 is based on the work published under the title: "Age gradients throughout the Galaxy with long-period variables" in Monthly Notices of the Royal Astronomical Society; Grady, Be- lokurov, and Evans (2019). • Chapter 3 is based on the work published under the title "Age demographics of the Milky Way disc and bulge" in Monthly Notices of the Royal Astronomical Society; Grady, Belokurov, and Evans (2020). • Chapter 4 is based on the work published under the title "Magel- lanic Mayhem: Metallicities and Motions" in The Astrophysical Journal; Grady, Belokurov, and Evans (2021). It is not substantially the same as any that I have submitted, or, is being concurrently submitted for a degree or diploma or other qual- ification at the University of Cambridge or any other University or similar institution. I further state that no substantial part of my disser- tation has already been submitted, or, is being concurrently submitted for any such degree, diploma or other qualification at the University of Cambridge or any other University of similar institution. It does not exceed the prescribed 60, 000 word limit. Cambridge, March 2021 James Richard Grady 1 Where appropriate, I maintain the use of the proverbial "we" so as to be consistent with the published works that form this thesis. For Dad ACKNOWLEDGMENTS I would like to sincerely thank my supervisors, Prof. N. Wyn Evans and Prof. Vasily Belokurov, for their continual guidance throughout my time studying in Cambridge. Both have provided me with a great deal of advice and encouragement in my studies, layered with good humour. I would also like to thank all members of the Streams group. Their suggestions and comments have always been fruitful and the group meetings have provided stimulating topics of discussion and inquiry. I thank Sergey Koposov for maintaining the Whole Sky Database, a truly invaluable resource for any astronomer. I further thank the ad- ministrative staff within the Institute for their prompt and effective support. Acknowledgement is owed to the Science and Technologies Facilities Council who provided the funding for my research. I would like to thank my fellow students at the Institute; I have made some great friends. Many an evening we have spent laughing and conversing in some of Cambridge’s fine pubs! A necessary anti- dote to the rigours of research. Further thanks goes to Corpus Christi College for hosting me dur- ing my time here in Cambridge. I have enjoyed partaking in the unique collegiate life offered by this University. It has also been a pleasure to represent the College in my sporting efforts, having had great fun in turning out regularly for the football and cricket teams alongside a thoroughly enjoyed spell of rowing. Above all, I thank my family. Unfailing in your love and support, you have kept me going. Thank you. vii CONTENTS 1 introduction1 1.1 The Milky Way in Context 1 1.2 The Galactic Bulge 2 1.2.1 Bulge Morphology 3 1.2.2 Stellar Ages in the Bulge 4 1.2.3 Bulge Metallicity Distribution 5 1.2.4 Bulge Kinematics 6 1.3 The Galactic Disc 8 1.3.1 Disc Dichotomy 9 1.3.2 Formation of the Discs 11 1.3.3 Disc Substructure 13 1.4 The Magellanic Clouds 14 1.4.1 Large Magellanic Cloud 15 1.4.2 Small Magellanic Cloud 17 1.4.3 Magellanic Interactions 17 1.5 Long Period Variables 19 1.5.1 What is a Long Period Variable? 19 1.5.2 Useful characteristics of Long Period Variables 20 1.6 Thesis Structure 21 2 age gradients throughout the galaxy with long period variables 23 2.1 Data 23 2.1.1 Mira Selection 24 2.1.2 O-rich LPV Distributions 27 2.1.3 Distance Determination and Validation 28 2.2 Galactic Age Gradients 31 2.2.1 Miras outside of the Galactic disc 36 2.2.2 Satellite Galaxy and Globular Cluster Associa- tions 41 2.3 Summary 44 3 age demographics of the milky way disc and bulge 47 3.1 Data 47 3.1.1 Selecting Miras 48 3.1.2 Miras in the Large Magellanic Cloud 49 3.1.3 Galactic Sample 50 3.2 Stellar Density Profiles 52 3.2.1 Modelling the Galactic Disc and Bulge 52 3.2.2 Simulated data 56 3.3 Results 58 ix x contents 3.3.1 Miras in the Disc 59 3.3.2 Miras in the Bulge 65 3.4 Summary 72 4 magellanic mayhem: metallicities and motions 75 4.1 Data 75 4.2 Regression Analysis 80 4.2.1 Regression Performance 83 4.2.2 Regression Predictions 83 4.3 Metallicity Maps 86 4.3.1 Metallicity Gradients 91 4.3.2 Slicing the Clouds by Metallicity 98 4.3.3 SMC shape and disruption 102 4.3.4 Old stellar Bridge 108 4.4 Summary 112 5 conclusions and outlook 115 5.1 Conclusions 115 5.2 Outlook 117 a appendix 121 a.1 Mira Cluster Associations 121 a.2 Density profile likelihood 122 a.3 Model M2 Residuals 123 bibliography 125 LISTOFFIGURES Figure 1.1 APOGEE giant abundance trends 10 Figure 1.2 Magellanic main sequence stars and red gi- ants. 16 Figure 1.3 Epoch photometry of Gaia LPVs. 20 Figure 2.1 Selection criteria for O-rich LPV candidates from the CRTS and ASAS-SN data sets. 25 Figure 2.2 Period-luminosity diagram for Gaia LPVs in the LMC. 26 Figure 2.3 Visual amplitude histograms of selected and rejected O-rich candidates. 27 Figure 2.4 Spatial distributions of O-rich LPVs in CRTS and ASAS-SN. 28 Figure 2.5 Distance estimate validation using the LMC and the Sgr Stream. 29 Figure 2.6 Location of O-rich LPVs in Galactocentric cylin- drical coordinates. 30 Figure 2.7 Dependence of Mira age with period using Galac- tic and Magellanic clusters. 31 Figure 2.8 Spatial footprint of O-rich LPVs as a function of pulsation period. 32 Figure 2.9 Profiles of median LPV pulsation period through the Galaxy. 33 Figure 2.10 Period profiles of ASAS-SN stars through the disc at different heights from the plane. 34 Figure 2.11 Decomposition of LPVs sub-groups in on-sky projection. 37 Figure 2.12 Difference in spatial extent between short and long period LPVs. 38 Figure 2.13 Distribution of old, short-period LPV stars in the Milky Way halo as viewed by CRTS. 40 Figure 2.14 Great circle counts of LPVs. 41 Figure 2.15 Dwarf galaxy and globular cluster associations. 42 Figure 3.1 Gaia+2MASS LPVs in Galactic projection. 48 Figure 3.2 Mira selection in Gaia DR2. 49 Figure 3.3 Mira period-luminosity diagram. 50 Figure 3.4 Miras in the Galaxy at different periods. 51 Figure 3.5 Luminosity function of Mira sample. 54 Figure 3.6 Simulated data modelling. 57 xi xii List of Figures Figure 3.7 Marginalised posterior distributions for den- sity model parameters. 58 Figure 3.8 Evolution of recovered model parameters as a function of Mira pulsation period 60 Figure 3.9 Model residuals in Galactocentric (X, Y) pro- jection for disc+bulge.
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