A MID-INFRARED STUDY OF BLUE COMPACT DWARF GALAXIES A Dissertation Presented to the Faculty of the Graduate School of Cornell University in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy by Yanling Wu January 2009 c 2009 Yanling Wu ALL RIGHTS RESERVED A MID-INFRARED STUDY OF BLUE COMPACT DWARF GALAXIES Yanling Wu, Ph.D. Cornell University 2009 Blue Compact Dwarf Galaxies (BCDs) are characterized by their blue optical colors, low luminosities and small sizes. Typically BCDs are found to have low, subsolar metallicities, and they are proposed to be local analogs to the primeval galaxies in the early universe. The unprecedented sensitivity of the Spitzer Space Telescope has enabled, for the first time, the detection of a large sample of these low luminosity systems. In this dissertation, I will present my work employing Spitzer data to study the dust properties and chemical evolution of blue compact dwarf galaxies. Using Spitzer/IRS low-resolution spectra (λ/δλ ∼ 100), I investigated the presence/absence of the Polycyclic Aromatic Hydrocarbon (PAH) features in low- metallicity galaxies. After exploring different physical parameters, I find that the strength of PAH emission is best correlated with a combination of forma- tion and destruction effects. In addition, using the high-resolution spectroscopy (λ/δλ ∼ 600), I study the elemental abundances of neon and sulfur in BCDs and compare with the optical studies. I find that the infrared derived elemental abun- dances generally agree with the optical results, though the neon abundances are slightly higher from our study. I also analyze the FIR/radio correlation in BCDs and expand the discussion to the mid-IR. I find that the infrared luminosities are well correlated with the radio luminosities even in these low luminosity systems. A study on the spectral energy distribution (SED) and star formation rates (SFRs) in low-luminosity star-forming galaxies is also included in this thesis. The SEDs appear to be warmer than normal star-forming galaxies, and their SFRs vary over an order of magnitude. Finally, I also present a case study on the famous BCD IZw18, showing the first mid-infrared spectrum for this galaxy. BIOGRAPHICAL SKETCH Yanling Wu was born on the 25th of June, 1980 to Qingmin Wu and Wenqin Li in the beautiful city of Suzhou, Jiangsu province, P.R China. There she spent the first 18 years of her life before she moved to Beijing for college. Yanling spent four years in Tiankuqian primary school, and then transferred to Dongzhongshi primary school, where she started to learn English. This turned out to be a wise decision because starting from that year, Suzhou Middle School reopened its junior high school section, and without knowing any English, Yanling would not be able to pass their entrance exams. There were four rounds of exams and Yanling ranked first in each of the rounds. The years in Suzhou Middle School were unforgettable for Yanling. She became a member of the international class, where she met many talented fellow students. Yanling was very thankful for this healthy environment of competition. She became interested in physics with the help of her physics teacher, Yiran Wang, which finally led her to decide to study astronomy in college. She was also very grateful to her English teacher, Yi Guan, without whom Yanling’s English would be inadequate to survive in the US. After spending six years in Suzhou Middle School, Yanling joined the astronomy department of Peking University as an undergraduate student, where she received her Bachelor’s Degree in physics in 2003. She did her undergraduate thesis under the supervision of Professor Zuhui Fan on the Sunyaev Zel’dovich effect, and that work was published in the Astrophysical Journal the same year. Yanling decided to go to graduate school in the United States. Because of its prestigious infrared program, Yanling finally picked Cornell as the place to spend the next five years, without knowing how much lovely snow she would enjoy while in Ithaca. The Spitzer Space Telescope was successfully launched in the same week iii as Yanling arrived at Cornell and she became a member of the Spitzer/IRS team half a year later, working with Professor Jim Houck. There she spent the last fours of her graduate program. Her work on the mid-infrared properties of blue compact dwarf galaxies using Spitzer observations culminated in this dissertation. iv To my parents Qingming Wu and Wenqin Li. v ACKNOWLEDGEMENTS Even though the winters in Ithaca are long, the last five years still passed quickly. Now I have a dissertation in my hand and I cannot wait to thank all the wonderful people who have helped me through my Ph.D. I would like to thank my advisor, Jim Houck, without whom I would not have the great opportunity to explore the beauty of infrared astronomy. Jim, thank you for introducing me to those little galaxies that ended up as the objects I study for my dissertation. I am very thankful to the GTO time you gave me for my Ph.D study, as well as the valuable advice on my research. My road to becoming an infrared astronomer would not be possible without Jim’s help. My five years as a Ph.D candidate would have much less fun if I haven’t met my co-advisor, Vassilis Charmandaris. Vassilis, thank you so much for being my advisor, and more importantly, as a life-long friend. I started with little under- standing of astronomy and now I am finishing up my dissertation. This would not happen without your help. I still remember that in the beginning I did not dare to ask questions because I worried about my ignorance showing and it was your patience and kindness that helped me to open my mouth and talk about science. I like your style of being a very hands-on advisor. You always gave me suggestions when I was at a lost, and you have encouraged me to keep going when I need it. It is very fortunate for me to have you as my co-advisor during my last five years and I hope that our collaboration will go on forever. This is a statement based solely on your science merit, not on your charm, though it is a solid fact that you are the most handsome man in the astronomy department. I mus thank Jeronimo Bernard-Salas, from whom I learned how to derive the elemental abundances using the infrared data. Jero not only explained the method to me very patiently, but also generously shared his code, and most importantly, vi taught me how to think critically when dealing with science questions. I also have to tell you that you are astonishingly handsome which makes working with you a very pleasant experience. There are so many people on the IRS team that I have benefited from (in al- phabetical order): Don Barry, Jeronimo Bernard-Salas, Bernhard Brandl, Vassilis Charmandaris, Daniel Devost, Duncan Farrah, Elise Furlan, Shannon Gutenkunst, Peter Hall, Lei Hao, Terry Herter, James Higdon, Sarah Higdon, Jim Houck, David Levitan, Jason Marshall, Laurie McCall, Greg Sloan, Henrik Spoon, Kevin Uchida, Dan Weedman and David Whelan. I would like to thank all of you. The IRS group is a like a big family and I feel your warmness. I suspect this might be the primary reason that help me to survive in Ithaca, where the temperature could drop to -30F sometime every winter. I would like to give my special thanks to Shannon, who shared an office with me the past three years. Shannon, thank you very much for putting up with me, always answering my questions quickly and caring about me when I am in blue. I would also have to mention my former officemate Sabrina Stierwalt, with whom I spent the first year in gradschool, and in the US. Sabrina, thank you so much for making me very comfortable even though this is the first time I have ever been to a foreign country. Please also take my thanks to your family, with whom I spent two Thanksgivings. Many thanks to Patrick, Ryan and Suniti, who shared an office with me in my second year. Patrick, you still owe me the story of how you started to call me a duck when were were in Room 110. I am very gratetful to the American Astronomical Society, which provided me travel grants to cover the expense of attending two conferences in China. I would like to thank Yu Gao for organizing these meetings, which gave me the opportunity to explore the development of astronomy in China, as well as meeting vii many scientists. I met Rob Kennicutt at a conference in Lijiang, China, in the summer of 2005. Rob, thank you so much for making friends with a not-know-much second-year graduate student. That encouraged me and I have learned so much from you, sharing thoughts on many things, not only research, but also life. I would also like to thank Rob for bringing me into the Local Volume Legacy Survey (LVLS) team. Thanks to all the people on LVLS. Even though we have just started, I have to say that it is my great pleasure to be working with the team. I would also like to give my thanks to Jiasheng Huang, my great mentor and friend in astronomy, as well as gastronomy. Jiasheng, thank you so much for the tremendous amount of support you gave me in the darkest part of my life. I am so glad that I have you as a friend during that period.