THE OUTERMOST HII REGIONS OF NEARBY GALAXIES by Jessica K. Werk A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Astronomy and Astrophysics) in The University of Michigan 2010 Doctoral Committee: Professor Mario L. Mateo, Co-Chair Associate Professor Mary E. Putman, Co-Chair, Columbia University Professor Fred C. Adams Professor Lee W. Hartmann Associate Professor Marion S. Oey Professor Gerhardt R. Meurer, University of Western Australia Jessica K. Werk Copyright c 2010 All Rights Reserved To Mom and Dad, for all your love and encouragement while I was taking up space. ii ACKNOWLEDGMENTS I owe a deep debt of gratitude to a long list of individuals, institutions, and substances that have seen me through the last six years of graduate school. My first undergraduate advisor in Astronomy, Kathryn Johnston, was also my first Astronomy Professor. She piqued my interest in the subject from day one with her enthusiasm and knowledge. I don’t doubt that I would be studying something far less interesting if it weren’t for her. John Salzer, my next and last undergraduate advisor, not only taught me so much about observing and organization, but also is responsible for convincing me to go on in Astronomy. Were it not for John, I’d probably be making a lot more money right now doing something totally mind-numbing and soul-crushing. And Laura Chomiuk, a fellow Wesleyan Astronomy Alumnus, has been there for me through everything − problem sets and personal heartbreak alike. To know her as a friend, goat-lover, and scientist has meant so much to me over the last 10 years, that confining my gratitude to these couple sentences just seems wrong. I was born academically in the Astronomy Department of Wesleyan University. I have only fond memories of the place and people that comprised it during those Golden Years. And if I was born academically at Wesleyan, then I learned to walk in the Univer- sity of Michigan Astronomy Department, a truly wonderful, engaged, and supportive community of scientists, students, and staff. Sarah Lloyd in the main office man- aged to be so kind while doing some of the most thankless work, and I thank her deeply for answering all my mundane questions ranging from health insurance to travel reimbursement to “detachment” issues with a smile. I hold all the graduate students who overlapped with me in such high regard, and can’t wait to see them at iii future conferences and to cite their papers, and to perhaps work with them. And of course, specifically, to Sarah Ragan, Janet Colucci, Laura Ingleby, Jen Blum, Becky Stanek, Matt Walker, Jeffrey Fogel, Tom Brink, Scott Cameron, Ajay Tannirkulam, Eli Rykoff, and Jun Ji, a huge thank you, for everything, but mostly for keeping me sane. Sarah and Janet get another whole line to themselves here, because, well, they’re just so awesome. All of the professors in the Department were excellent to me, and taught me so much both inside and outside of the classroom. In particular, Sally Oey, who continues to involve me in her group meetings, has been so supportive during my move to Columbia University, a pleasure to work with, and a great mentor. Nuria Calvet often went above and beyond her role of graduate student mentor, and is a big part of the reason why I’m not currently suffering from wisdom tooth pain. And all of my dissertation committee members have offered me valuable insight over the last few years for which I am grateful. I am grateful to Mario Mateo in particular, who took over as my local committee chair and thesis advisor in the aftermath of my “detachment, ” and who has offered sound scientific advice, always, during my time at the University of Michigan. The Astronomy Department at Columbia University has been most hospitable during my two-year stay as a Non-Degree-Seeking Visiting Scholar. Everyone, from faculty to staff to students, has been welcoming. In particular, Mildred Garcia and Trudy Reyes made sure my somewhat complicated status didn’t get in the way of health insurance and stipend. Jennifer Donovan-Meyer, Stephanie Tonnesen, and Sarah Tuttle have been true friends during these thesis-writing days, almost making it fun. Jana Grcevich, another Michigan transplant, is one of the smartest, most wonderful people I have ever met, and I am so glad to have worked with her. Joshua Peek inspires me everyday with his enthusiasm for astronomy and new, exciting ideas. And Jacqueline van Gorkom never ceases to impress me with her boundless energy for astronomy, effortless insight, and continued kindness. The research presented in this Thesis has made extensive use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and iv Space Administration. I am grateful for its existence, for sure. A National Science Foundation Graduate Student fellowship supported me financially for 3 years while working on this project, and afforded me many opportunities to travel and discuss my research with other scientists. I am definitely fortunate to have received this fellowship. Outside of the astronomical community, my family and friends were endlessly supportive and genuinely curious about my research, a real boon during the requisite soul-searching that accompanies the decision to become an academic. And Nicholas Ranallo put up with more than his share of random but impassioned exclamations about astrophysical processes on Saturday afternoons – I can’t wait until our Saturday afternoons are once again filled with the HP and GD instead. Not soon enough... Gerhardt Meurer has managed to teach me so much, though we’ve never been in the same place for more than one week. His scientific advice, moral support, and guidance over the last six years have been invaluable to me. Any thanks I could offer here would be totally inadequate. And everyday, I feel fortunate beyond words to be working with Mary Putman. I just hope that someday I can be half as insightful, thoughtful, and brilliant as she is. Without a doubt, she is exactly the sort of astronomer I aspire to be. v PREFACE This work presented here relies upon the data from no fewer than 10 observing runs on various telescopes – the MDM 2.4-m, Magellan Baade 6.5-m, Gemini-North 8-m, VLA, HST – discussions with scientists at 9 astronomical conferences, and col- laborative efforts with astronomers all over the world. I am indebted to so many of them for insightful suggestions and comments on this work in various forms – pa- per drafts, power-point presentations, and conference posters. Emma Ryan-Weber, Robert Kennicutt, Janice Lee, Diedre Hunter, Bill Vacca, David Thilker, Daniel Han- ish, Ken Freeman, Andrey Kravtsov, Mederic Boquien, Joel Bregman, Nate Bastien, Rupali Chandar, Hans Zinneker, Carolina Kehrig, Joss Bland-Hawthorn, Carsten Weidner, Ron Allen, Xavier Prochaska, Barry Madore, Alan Dressler, Liese van Zee, Kevin Croxall, Eric Keto, Iraklis Konstantopoulos, Jay Strader, David Schiminovich, all those mentioned in the Acknowledgments, and more have all contributed a great deal in helping me interpret the results of this thesis. Chapters 2, 3, and 4 of this thesis, though slightly modified from their original versions, were published previously in both The Astronomical Journal and The Astro- physical Journal. They are reproduced here (altered slightly to fit the thesis format) with permission from the AAS. The full citation for each article is: Chapter 2 - Werk, J. K., Putman, M. E., Meurer, G. R., Ryan-Weber, E. V., Kehrig, C., Thilker, D. A., Bland-Hawthorn, J., Drinkwater, M. J., Kennicutt, R. C., Wong, O. I., Freeman, K. C., Oey, M. S., Dopita, M. A.; Doyle, M. T., Ferguson, H. C., Hanish, D. J., Heckman, T. M., Kilborn, V. A., Kim, J. H., Knezek, P. M.; Koribalski, B., Meyer, M., Smith, R. C., Zwaan, M. A. “Outlying HII Regions in HI-Selected Galaxies.” 2010, AJ, 139, 279 Chapter 3 - Werk, J. K., Putman, M. E., Meurer, G. R., Oey, M. S., Ryan- vi Weber, E. V., Kennicutt, R. C., & Freeman, K. C. “Isolated OB Associations in Stripped HI Gas Clouds.” 2008, ApJ, 678, 888 Chapter 4 - Werk, J. K., Putman, M. E., Meurer, G. R., Thilker, D. A., Allen, R. J., Bland-Hawthorn, J., Kravtsov, A., & Freeman, K. “The Metal-enriched Outer Disk of NGC 2915.” 2010, ApJ, 715, 656 vii CONTENTS DEDICATION......................................... ..... ii ACKNOWLEDGMENTS .................................... iii PREFACE............................................ ..... vi LISTOFFIGURES ...................................... xiii LISTOFTABLES ....................................... xix ABSTRACT ........................................... xxi CHAPTER 1 Introduction ......................................... 1 1.1 MakingStarsoutofGasinGalaxies ................... .. 2 1.1.1 The Spatially-Resolved and Low-Density Star Forma- tionLaws ................................. 5 1.1.2 Star Formation in the Outskirts of Galaxies . 6 1.2 The Stellar Populations of HII Regions . .... 8 1.2.1 Integrated Properties of OB Associations . 10 1.2.2 Distribution of Masses in Outlying OB Associations . 11 1.3 HII Regions as Tracers of Gas-Phase Metallicity . ...... 14 1.3.1 The Chemical Evolution of the Outermost Gas . 15 1.3.2 The Mass-Metallicity Relation for Galaxies . 16 1.4 InThisThesis... ................................. .. 18 1.4.1 The Frequency and Nature of Outlying HII Regions . 19 1.4.2 The Underlying Stellar Populations of Outlying HII Re- gions .................................... 20 1.4.3 The Metal Abundances of Outlying HII Regions . 20 viii 2 The Frequency and Nature of Outlying HII Regions in HI- Selected Galaxies ..................................... 22 2.1 Introduction.................................... .. 22 2.2 SINGGdata...................................... 23 2.3 SampleofELdots .................................. 25 2.3.1 SelectionCriteria............................ 25 2.3.2 SampleOverview............................ 28 2.3.3 SelectionEffects ............................ 29 2.4 SpectroscopicFollow-Up .......................... ... 32 2.5 ComparisonwithdeepGALEXdata..................... 37 2.5.1 GALEX Morphology and Outlying HII Regions .