A NEW PERSPECTIVE ON GALAXY EVOLUTION FROM THE LOW DENSITY OUTSKIRTS OF GALAXIES by AARON E. WATKINS Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Astronomy Dissertation Adviser: Dr. J. Christopher Mihos CASE WESTERN RESERVE UNIVERSITY August, 2017 CASE WESTERN RESERVE UNIVERSITY SCHOOL OF GRADUATE STUDIES We hereby approve the dissertation of Aaron E. Watkins candidate for the degree of Doctor of Philosophy. Committee Chair J. Christopher Mihos Committee Member Paul Harding Committee Member Stacy McGaugh Committee Member Heather Morrison Committee Member Steven A. Hauck Dissertation Defense June 19, 2017 Dedicated to the three most important people in my life: Mom, Dad, and Nathaniel Contents Title 1 Signature Sheet ii Table of Contents iii List of Tables vii List of Figures viii Preface x Acknowledgements xi Abstract xiii 1 Introduction 1 1.1 Tidal Interactions and Environment . .4 1.2 Secular Evolution and the Outer Disk . .9 1.3 Star Formation At Low Density . 14 1.4 Summary, and the Aims of This Thesis . 18 2 Searching For Diffuse Light in the M96 Galaxy Group 20 2.1 Abstract . 20 2.2 Introduction . 22 2.3 Observations and Data Reduction . 28 2.3.1 Observations . 28 2.3.2 Data Reduction . 29 2.4 Large Scale Diffuse Light . 32 2.5 Individual Galaxies . 41 2.5.1 M105 . 42 2.5.2 NGC 3384 . 45 2.5.3 M96 . 47 iv 2.5.4 M95 . 50 2.6 Discussion . 52 2.6.1 The Origin of the Leo Ring . 52 2.6.2 The Lack of Intragroup Light . 57 2.7 Summary . 60 2.8 Acknowledgments . 62 3 Deep Imaging of M51: a New View of the Whirlpool’s Extended Tidal Debris 63 3.1 Abstract . 63 3.2 Introduction . 65 3.3 Observations and Analysis . 67 3.4 Results . 69 3.4.1 Morphology and Color of Tidal Features . 69 3.4.2 Comparison to Simulations . 75 3.5 Acknowledgements . 77 4 The Red and Featureless Outer Disks of Nearby Spiral Galaxies 78 4.1 Abstract . 78 4.2 Introduction . 80 4.3 Observational Data . 85 4.3.1 Deep Optical Imaging . 85 4.3.2 Ancillary Multiwavelength Data Sets . 87 4.4 Analysis Techniques . 88 4.4.1 Surface Brightness and Color Profiles . 88 4.4.2 Fourier Analysis . 90 4.5 Individual Galaxies . 91 4.5.1 M94 (NGC 4736) . 96 4.5.2 M64 (NGC 4826) . 99 4.5.3 M106 (NGC 4258) . 105 4.6 Discussion . 112 4.6.1 Outer Disk Stellar Populations . 113 4.6.2 Environmental Influences . 116 4.7 Summary . 120 4.8 Acknowledgements . 122 4.9 Appendix: ON THE BURRELL SCHMIDT PSF . 122 5 HII Regions and Diffuse Ionized Gas Throughout the M101 Group: Only the Intensity Changes 126 Abstract 127 5.1 Introduction . 128 5.2 Observations and Data Reduction . 133 5.2.1 Observations . 133 v 5.2.2 Data Reduction . 135 5.2.3 GALEX data . 140 5.2.4 Background/Foreground Contamination . 140 5.3 HII Region Photometry . 141 5.3.1 Extinction Correction . 142 5.3.2 Photometry . 144 5.3.3 Statistical Analysis . 150 5.3.4 The Unchanging Nature of HII Regions . 154 5.4 Diffuse Ionized Gas . 156 5.4.1 Isolating the DIG . 157 5.4.2 Results . 158 5.5 Discussion . 163 5.5.1 The Connection Between Star Formation and the DIG . 164 5.5.2 On the Observed Trends of Integrated FHα=FFUV ........... 167 5.5.3 The M101 Group As a Case Study . 170 5.6 Summary . 173 5.7 Acknowledgements . 175 6 Summary and Future Work 176 6.1 Major Results of This Dissertation . 176 6.2 Directions for Future Work . 179 Bibliography 183 vi List of Tables 2.1 Properties of Streamlike Features in the M96 Group . 30 3.1 Photometry of Tidal Debris in M51 . 67 4.1 M94, M64, and M106 Galaxy Properties . 92 5.1 Results of Levene’s Test Trials . 153 5.2 Integrated Properties of M101 Group Galaxies . 168 vii List of Figures 2.1 False-color image of the M96 Group . 25 2.2 Binned and labeled broadband mosaics of the M96 Group . 33 2.3 HI contours overlaid on the M96 Group V-band mosaic . 34 2.4 Schematic of M96 Group features . 36 2.5 GALEX and V-band images of Leo Ring star-forming complexes . 40 2.6 M105 surface brightness and color profiles . 43 2.7 Residuals of model-subtracted NGC 3384 & M105 . 44 2.8 NGC 3384 surface brightness and color profiles . 48 2.9 M96 surface brightness and color profiles . 50 2.10 M95 surface brightness and color profiles . 52 3.1 Rescaled B-band image of M51 . 66 3.2 M51 pixel-to-pixel colormap . 68 3.3 M51 21cm imaging overlay . 71 3.4 Surface brightness and color profile of a wedge across M51 . 73 4.1 M94 multiwavelength comparison . 94 4.2 M94 surface brightness, color, and Fourier analysis profiles . 95 4.3 M64 multiwavelength comparison . 100 4.4 M64 surface brightness, color, and Fourier analysis profiles . 101 4.5 M106 multiwavelength comparison . 106 4.6 M106 surface brightness, color, and Fourier analysis profiles . 107 4.7 NGC 4248 closeup . 108 4.8 Burrell Schmidt B- and V-band PSF radial profiles . 124 5.1 Hα difference image mosaic of the M101 Group . 134 5.2 Tentative diffuse plume northeast of M101 . 138 5.3 Extinction correction comparison . 143 5.4 HII region fluxes and FHα=FFUV ratios, M101 . 146 5.5 HII region fluxes and FHα=FFUV ratios, NGC 5474 . 147 5.6 Inner disk/outer disk boundary in M101 . 148 5.7 Evolution of FHα=FFUV in Starburst99 . 151 5.8 Statistical comparisons of FHα=FFUV across the M101 Group . 151 viii 5.9 Observed and model distributions of FHα=FFUV in M101 . 155 5.10 M101 and NGC 5474 DIG reprojections . 159 5.11 Radial profiles of FHα=FFUV in the DIG . 160 5.12 HII region flux surface density correlation with DIG FHα=FFUV ....... 162 5.13 HII region flux surface density against HI and H2 column densities . 165 ix Preface This dissertation presents the culmination of 6 years’ worth of my work as a graduate student. The material presented in Chapters 2, 3, and 4 has previously been published as (respectively): Watkins, Mihos, Harding, & Feldmeier (2014), “Searching for Diffuse Light in the M96 Galaxy Group”, ApJ, 791, 38; Watkins, Mihos, & Harding (2015), “Deep Imaging of M51: a New View of the Whirlpool’s Extended Tidal Debris”, ApJL, 800, L3; and Watkins, Mihos, & Harding (2016), “The Red and Featureless Outer Disks of Nearby Spiral Galaxies”, ApJ, 826, 59. Though each was authored by several people, I am the primary author on each and hence am responsible for the majority of the research, analyses, and written content therein. x Acknowledgements Maybe it’s an obvious place to start, but I want to first thank my adviser, Chris Mihos. I started working with him my first day here at CWRU, and, even in the face of occasional bouts of extreme grumpiness in my bumbling early days, he stuck with me and I stuck with him for the next six years. I know that without him, I’d be a vastly sloppier speaker, writer, and scientist. I also want to thank Paul Harding, who is really my other adviser. Paul taught me to pay attention to the nitty-gritty and to never take anything for granted. Until you’ve sat with him for a full 6 hours as he makes minute adjustments to the readout parameters in the CCD controller software, you have no idea just how sensitive an instrument a giant piece of polished glass and metal can be. And that kind of insight is invaluable. I would be remiss in neglecting to thank everyone else in this small department as well, from.