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C 2012 by Ki Jeong Yim c 2012 by Ki Jeong Yim. THE INTERSTELLAR MEDIUM AND STAR FORMATION IN EDGE-ON GALAXIES BY KI JEONG YIM DISSERTATION Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Astronomy in the Graduate College of the University of Illinois at Urbana-Champaign, 2012 Urbana, Illinois Doctoral Committee: Assistant Professor Tony Wong, Chair Professor You-Hua Chu Professor Charles Gammie Associate Professor Leslie Looney Abstract This thesis presents a study of the vertical disk structure and star formation in four edge-on spiral galaxies (NGC 891, 4157, 4565, and 5907) observed in CO with BIMA/CARMA, H I with VLA, and IR (3.6 and 24 µm) with Spitzer. We first de- rive the radial density distributions for the edge-on galaxies using our PVD method for the radio data and GIPSY task RADPROF for IR data. The derived radial pro- files are used to verify the correlation between star formation rate (SFR) and gas surface densities (ΣH2 and Σgas), and between the ΣH2 /ΣHI ratio and the hydrostatic midplane pressure that have been found in face-on galaxies by previous studies. In addition, we examine the relationship between SFR and the molecular gas based on a pixel-by-pixel method. In order to estimate the volume densities of gas and stars, we measure the disk thickness as a function of radius, taking into account projection effects for less edge- on galaxies (NGC 4157, 4565, and 5907) by determining and correcting for the disk inclination. We also infer the vertical velocity dispersions with radius using the de- rived volume densities and disk thicknesses. Overall, the disk thicknesses increase with radius and the velocity dispersions decrease as a function of radius. We test the importance of gravitational instability parameter Q in determining massive star forming regions and the importance of interstellar gas pressure in control- ling the ρH2 /ρHI ratio for two cases: varying and constant velocity dispersions. The Q parameter does not show a clear correlation with massive star formation although Q using varying velocity dispersion shows marginal instability in some galaxies. Both velocity dispersion models seem to show a well defined power-law relationship be- tween the pressure and the ρH2 /ρHI ratio, although their relationship is tighter for the constant values. We conclude that the ρH2 /ρHI ratio is more closely related to the gas volume density than to the pressure. ii To my family. iii Acknowledgments It is a pleasure to acknowledge the various kinds of help and support I have received during the completion of this thesis. First, my most sincere thanks go to my advisor, Tony Wong, for his dedication, advice, and guidance. He always gave me good ideas whenever I faced problems and issues engaged my research. I have been fortunate to meet such a great advisor. This dissertation has been made up with his countless valuable comments and suggestions. I also want to express my gratitude for his written contribution in Section 2.6.2 (relating to a theoretical estimate of the SFR) and Appendix B. I would like to thank You-Hua Chu, Charles Gammie, Leslie Looney, Thijs van der Hulst, and Chris Howk for their very helpful and insightful comments on this work. I also wish to record my appreciation to Rich Rand for detailed and invaluable comments on Chapter 3 and the support at the University of New Mexico for two weeks. Richard Crutcher provided me a enjoyable project for the first summer as a graduate student and Athol Kemball offered me financial support for a year and great opportunity to learn about VLBA Data Reduction using AIPS. Many thanks to Rui Xue who helped me to reduce many H I data using CASA and contributed paragraphs in Section 3.2.2 (H I reduction). It was a pleasure to share a moment with David Rebolledo over the group meeting and conference trips. I am also grateful to all of the faculty, staff, and graduate students at the Astronomy Department for providing great lectures, nice support, and friendly environment. Work on this dissertation has been supported over the years by the CARMA participating institutions and the National Science Foundation under cooperative agreement AST-0838226 and by a Spitzer Cycle-5 data analysis award from NASA. Finally, I would like to give particular thanks to my wonderful parents and sister Jahoon for their tangible and intangible support. I also thank my daughter, Dyne, for letting me go to my office after dinner, while I was writing this thesis. And most of all, my gratitude goes to my husband, Woojin Kwon, who spent more time with Dyne than me and enjoyed a discussion with me about my work as well. It would not iv have been possible to complete this thesis in time without his dedication. v Table of Contents ListofTables ................................. viii ListofFigures................................. ix 1 Introduction ................................ 1 1.1 Edge-OnDiskGalaxies.......................... 1 1.1.1 Photometricproperties . 1 1.1.2 Models of vertical distribution . 2 1.2 TheStarFormationLaw ......................... 3 1.3 GravitationalInstability . 4 1.4 Molecular-to-Atomic Ratio and Pressure . ... 5 1.5 Edge-OnGalaxySample ......................... 6 1.5.1 NGC891 ............................. 7 1.5.2 NGC4157............................. 7 1.5.3 NGC4565............................. 7 1.5.4 NGC5907............................. 7 2 ACaseStudyofNGC891........................ 9 2.1 Introduction................................ 10 2.2 ObservationsandDataReduction . 12 2.3 Kinematics ................................ 14 2.3.1 Position-VelocityDiagrams. 14 2.3.2 RotationCurve .......................... 14 2.4 RadialDistribution ............................ 15 2.4.1 MolecularandAtomicGas. 16 2.4.2 Infrared: 3.6 and 24 µm ..................... 18 2.5 VerticalDistribution ........................... 21 2.5.1 Disk Thickness in Integrated Intensity . 21 2.5.2 Radial Variation in Disk Thickness . 22 2.5.3 Radial Variation in Vertical Velocity Dispersion . ..... 23 2.6 The H2/HIRatioandStarFormation. 25 2.6.1 Hydrostatic Midplane Pressure . 25 2.6.2 StarFormationRateandEfficiency . 26 2.6.3 GravitationalInstability . 28 2.6.4 Vertical Dependence of Gas Pressure . 29 vi 2.7 Discussion................................. 31 2.7.1 SFRandMidplanePressure . 31 2.7.2 QintheStarFormationDisk . 31 2.7.3 Interstellar Gas Pressure in Two Dimensions . .. 32 2.8 SummaryandConclusions . 33 3 Less Edge-On Galaxies NGC 4157, 4565, and 5907 . 50 3.1 Introduction................................ 50 3.2 ObservationsandDataReduction . 53 3.2.1 COObservations ......................... 53 3.2.2 H I Observations ......................... 54 3.2.3 IRObservations.......................... 55 3.3 Radial Surface Density Distributions . ... 56 3.3.1 MolecularandAtomicGas. 56 3.3.2 StarsandStarFormationRate . 57 3.4 RadialVariationinVerticalStructure . .... 59 3.4.1 Inclination and Gas Disk Thickness . 59 3.4.2 StellarDiskThickness . 62 3.4.3 Comparison of the Disk Thicknesses . 63 3.4.4 Midplane Volume Densities . 64 3.4.5 Vertical Velocity Dispersion . 65 3.5 StarFormation .............................. 66 3.5.1 Kennicutt-SchmidtLaw . 66 3.5.2 GravitationalInstability . 68 3.6 MoleculartoAtomicGasRatio . 70 3.7 SummaryandConclusions . 72 4 Conclusions................................. 94 5 FutureWork................................ 97 5.1 NGC4013................................. 97 5.2 Better Resolution of H I DataforNGC4157and5907. 98 5.3 StarFormationalongBarStructures . 98 5.4 FromEdge-OntoFace-OnGalaxies . 99 Appendix A Radial Distributions of H2 and H I ............102 A.1 Comparison between PVD and RADPROF . 102 A.2 GalaxyModels .............................. 103 Appendix B Approximations for Hydrostatic Pressure . 105 References ................................... 107 vii List of Tables 1.1 GalaxyProperties............................. 8 2.1 Models of Vertical Velocity Dispersions . .... 24 3.1 GalaxyProperties............................. 52 3.2 CO and H I ObservingParameters . 54 3.3 Parameters from fitting the radial variation of the scale heights . 64 3.4 Power-law indices for the star formation law . .... 67 viii List of Figures 2.1 COintegratedintensitymapofNGC891 . 35 2.2 Comparison with the single-dish data (IRAM) . ... 36 2.3 Spitzer 3.6 and 24 µmemissionfromNGC891. 36 2.4 Position-velocity diagrams of CO and H I ................ 37 2.5 Observational rotation curve from CO and H I ............. 38 2.6 Radial profiles of ΣH2 , ΣHI, and Σgas .................. 39 2.7 Exponential disk model fit to the 3.6 µmimage ............ 40 2.8 Single Gaussian fitting to the CO and H I integratedmaps . 41 2.9 Scaleheightsasafunctionofradius . .. 42 2.10 Vertical velocity dispersion as a function of radius . ......... 43 2.11 Midplane pressure as a function of radius and Rmol vs. the hydrostatic pressure .................................. 44 2.12 SFR surface density as a function of ΣHI, ΣH2 , and Σgas ........ 45 2.13 SFR surface density as a function of P0gas−only/σg and the midplane pressure .................................. 46 2.14 Star formation efficiency (SFE) vs. radius and Rmol vs. radius . 47 2.15 Qgas, Qstar and Qgas+star radialprofiles. .. .. 48 2.16 Dependence of gas pressure on z atdifferentradii . 48 2.17 ρH2 /ρHI vs. gas pressure at different heights . 49 3.1 COintegratedintensitymaps . 75 3.2 H I integratedintensitymaps . 76 3.3 Spitzer 3.6 µmImages .......................... 77 3.4 Spitzer 24 µmImages........................... 78 3.5 Vertically integrated position-velocity diagram
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