The Mapping and Modelling of Spiral Structure in the Milky Way :

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1 The Mapping and Modelling of Spiral Structure in the Milky Way : The Potential of the Interstellar Medium Lee James Summers, BSc(Hons), MSc(R). A Thesis submitted by Lee James Summers to the University of Exeter for the degree of Doctor of Philosophy in Physics, March, This thesis is available for Library use on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement. I certify that all material in this thesis which is not my own work has been identified and that no material has previously been submitted and approved for the award of a degree by this or any other University. Signed: Lee James Summers Date:

2 Abstract GENTLEMEN, - It was in 1864 that the first clues as to the nature of the space between stars were first identified in literature. These spaces became known as the interstellar medium. Any study of star formation must include an understanding of this interstellar medium (ISM) and its various component parts. Molecular clouds, dense regions of the ISM, are the sites where all known star formation is thought to occur. Hence, whenever an area containing young stars reside, it is assumed that one will also find a molecular cloud. Knowledge of these stellar birthplaces assist not only in models of stellar evolution, star formation potential, rate and e ciency - but also the ISM gives indications as to Galactic structure and the dynamics therein. Within this thesis I begin with an introduction and historical background of the field before detailing the research which was conducted. Firstly, I discuss a new model describing the spatial and kinematic structure of the Milky Way s spiral potentials; the Perseus arm, the Outer arm and the outer Scutum-Centaurus arm and also the kinematics of the streaming motions of the gas within them. Material associated with each of these arms is then extracted. Using the models and spiral arm maps derived, I present spatially convolved maps of each spiral arm region (Perseus, Outer and Scutum-Centaurus) at a constant linear scale. By minimising the biases inherent with angular observations of our Galaxy, this presents the data as an analogue of - and as such directly comparable to - extragalactic observations of spiral structure. Finally I present a series of analyses performed on the data and models; derivation of large-scale properties of the spiral arms (i.e. identification of where the arm is unconfused with fore- and back-ground emission, scale height, velocity dispersions, arm mass); dynamical analyses of the models; molecular cloud decomposition of the constant-linear-scale-maps. The findings are then compared with those in the Galactic and extra-galactic literature. I am, Gentlemen, Yours Faithfully, L. J. Summers (Abstract in the style of: Christiansen & Hindman (1952), the first detection of Galactic HI) 2

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4 Contents Abstract 2 Table of Contents 4 Declaration & Derived Works 9 Acknowledgements 10 List of Figures 11 List of Tables 16 List of Symbols 17 1 Introduction & Historical Background The Nature of the Interstellar Medium Dark Marks on a Light Sky : The Void Problem The Interstellar SchISM The Relative Densities of the ISM Colder, Warmer, Hotter : The Multi-Phase ISM The One Phase Medium The Two Phase Medium The Three and Five Phase Medium The Hierarchy of the ISM Di use Clouds Giant Molecular Clouds Dark Clouds Dense Molecular Clouds Chemical and Physical Mechanisms within the ISM The Photodissociation of Molecules

5 Contents Molecular Cloud Chemistry The Detection of Atomic Hydrogen Formation of Molecular Hydrogen Dust Grain Catalysis Gas Phase Reactions Formation & Detection of Carbon Monoxide Converting a CO Detection to a Column of H 2 : The X CO Factor Measuring Cloud Mass Star Formation Mechanisms General Overview of Low Mass Star Formation High Mass Star Formation Mechanisms Stellar Collision & Mergers Monolithic Collapse Competitive Accretion The Milky Way in the Cosmos Galaxy Morphology & Classification Elliptical Class Galaxies Spiral Class Galaxies Lenticular Class Galaxies Irregular Class Galaxies The Fingers of God E ect The Milky Way - A Spiral Galaxy? Milky Way Spatial Spiral Structure Milky Way Kinematic Spiral Structure Inner-Galaxy Distance Ambiguity Outer-Galaxy Distance Ambiguity Preliminary Work - The Mass:Age Relation of ISM:YSC Thesis Content & Overview Source Data & Observations The Ex-FCRAO CO NGPS The International Galactic Plane Survey (IGPS) Supplemental Spectral Data Surveys Stellar Cluster Samples & Catalogues The WEBDA Catalogue of Stellar Clusters A Catalogue of Optically Visible Open Clusters, Dias et al. (2010) Summary Modelling Galactic Structure Introduction to Chapter The Perseus Arm The Outer Arm... 84

6 Contents Source Data Defining the model - The Perseus Spiral Arm Modelling the Spatial Structure Modelling the Kinematic Structure Overview of `-v LS R Structure in the CO Maps Definition of the Kinematic Model Kinematic Model of the Perseus Arm - Fit to Maser Positions Kinematic Model of the Perseus Arm - Refinement by 12 CO Centroids Predicting the Perseus Arm Trajectory in the inner-galaxy Distance Estimation using the Shocked Motion Model Defining the Model - The Outer Spiral Arm Modelling the Spatial Structure Modelling the Kinematic Structure Kinematic Model of the Outer Arm - Fit to Tracer Locations Kinematic Model of the Outer Arm - Refinement to 12 CO Centroids Kinematic Model of the Outer Arm - Assuming a variable v shock Perseus Arm Retrospective - A variable v shock Musings on the `-v LS R Structure of the Outer Arm Velocity Gradient Reversal Within Spiral Arms Beyond the Outer Arm : Scutum-Centaurus Compensating for the Kinematic distance estimates Spatial fit to the Sct-Cen molecular emission Modelling the kinematic structure of Sct-Cen Coupling of the Spatial & Velocity Models Milky Way Velocity Field Summary The Perseus Arm The Outer Arm The Outer Scutum-Centaurus Arm Galactic Spiral Arm Cartography Introduction to Chapter Spiral Arm Velocity Centric Mapping The ACM Process Perseus Spiral Arm Outer Spiral Arm Outer Scutum-Centaurus Spiral Arm Common Resolution Mapping The CRM Process Perseus Spiral Arm Outer Spiral Arm Outer Scutum-Centaurus Spiral Arm

7 Contents Spiral Arm Mid-plane Function Mapping The AMF Process Summary of Spiral Arm AMF Data Properties of the Spiral Arm Regions The Perseus Arm The Nature of the Perseus Arm Perseus Arm Velocity Dispersion - vel Perseus Arm Scale Height & Mass Perseus Arm Transverse Velocity Dispersion - vel z The Outer Arm The Nature of the Outer Arm Outer Arm Velocity Dispersion - vel Outer Arm Scale Height & Mass Transverse Velocity Dispersion - vel z Postulating the Sct-Cen Arm Structure Summary The Perseus Arm The Outer Arm The outer Scutum-Centaurus Arm Galactic Dynamics & The Molecular Content Of The Outer Milky Way Introduction to Chapter Galactic Rotation Non-Flat Rotation Curves Estimation of Oort s Constants Comparisons with other determinations of Galactic Rotation Analysis of the AMF Data Maps The Perseus Arm AMF The Outer Arm AMF Cloud Decomposition & Analysis The Cloud Sample & Parameters Mass Spectrum Size - Line Width Relation Luminosity-Size Relation The Apparent Resolution Dependence of Derived Parameters Systematic Resolution E ects in Clump Finding Algorithms The New Size Definition, S 0 solo The N min Anomalies The Outer Spiral Arm Clump Finding Summary

8 Contents 8 6 Final Summaries & Conclusions Summary of Work Presented Spatial Fitting to Spiral Structure The Kinematic Shocked-Motion-Model Spiral Arm Mapping Routines Clouding and Clumpology Future Work & Research Bibliography & References 212 Appendix 225 A Proof of Formation time of Molecular Clouds B Convolution of two one-dimensional Gaussian functions C Preliminary Work - The Mass:Age Relation of ISM:YSC C.1 Molecular Clouds and Stellar Clusters C.2 Condensation of Molecular Material C.3 Measuring Cloud Mass C.4 Galactic Structure and Kinematics Galactic Rotation Curves Perseus Spiral Arm Spatial Structure - The Hou et al Model Perseus Spiral Arm Kinematic Structure - The Russeil et al Model 235 Perseus Spiral Arm Association C.5 Source Data Selection and Preparation Source Catalogue Data Maps The Cluster Sample C.6 Mass Results Mean Cluster Radius Median Cluster Radius Modification To Velocity Assignment C.7 Conclusions