Radiative Processes in Astrophysical Gases: From the Intergalactic and Interstellar Medium to Exoplanetary Atmospheres Thesis by Antonija Oklopčić In Partial Fulfillment of the Requirements for the degree of Doctor of Philosophy CALIFORNIA INSTITUTE OF TECHNOLOGY Pasadena, California 2017 Defended May 9, 2017 ii © 2017 Antonija Oklopčić ORCID: 0000-0002-9584-6476 All rights reserved iii Mojim roditeljima, bez čije ljubavi i podrške ne bih bila to što jesam. iv ACKNOWLEDGEMENTS Time that I spent as a graduate student at Caltech has been an amazing period in my life and I am grateful to everyone who has been a part of it. I would like to begin by thanking everyone who has helped me get here in the first place, especially my undergraduate advisor Vernesa Smolčić. I gratefully acknowledge the support from the Fulbright International Science & Technology Fellowship that I have received during the first three years of graduate school. Above all, I would like to thank my advisor, Chris Hirata, for his immense kindness and support throughout the past six years, and especially for giving me the freedom to pursue my own scientific interests, wherever they may take me. I greatly enjoyed working with Chris and learning from him—I am very honored and proud to be his student. I owe gratitude to Phil Hopkins for introducing me to the world of hydrodynamic simulations. It has been great fun and privilege to work with him and the rest of the FIRE collaboration. I am grateful to Phil and his large group of students and postdocs for many interesting discussions, group meetings, and informal workshops—I have learned so much from them. I thank the members of my thesis defense committee: Lynne Hillenbrand, for always being supportive of students and committed to their well-being, especially in her role as the option representative; Tony Readhead, for his generosity and kindness, as well as for introducing me to the wonders of radiative processes in Ay121; Dimitri Mawet, for his interest and enthusiasm. I am grateful to Kevin Heng, who was the first to encourage Chris and me to start thinking about problems related to exoplanet atmospheres. Ever since, Kevin’s input and advice have been of immense value for both my research and for choosing my career path. My graduate school experience would not have been the same without the amazing friends that I have acquired along the way. I am extremely grateful to Gwen Rudie, my friend and mentor from day one. Gwen has been an unlimited source of wisdom, compassion, and support in my life for the past six years and a real role model. Ben Montet, Allison Strom, Trevor David, and Yi Cao – you guys made taking classes, doing homework, and studying for the qual more fun than I ever thought it could be possible. I will forever cherish the memories of the adventures we had together— v camping in the Grand Canyon, wine tasting in Napa, enjoying the fireworks at a Star Trek-themed Dodgers game, the first-year Christmas challenge(s), and many more. Thank you for being such an important part of my life! Abhilash Mishra and Teja Venumadhav, my academic brothers, thank you for all the fun we had together in Pasadena, Columbus, and everywhere in between. And finally, many thanks to all graduate students in Caltech Astronomy and TAPIR, past and present, for countless lunches at Chandler, Thursday donuts and Friday beers, Halloween parties, grad formals, softball games, and ski trips. To my favorite fellow grad student—my husband Mislav—I cannot express how grateful I am to you for everything in our lives. I know I would not be where I am today if it hadn’t been for your love, support, and encouragement along the way. I love sharing my life with you and I am looking forward to the next big step ahead of us—Cambridge (Massachusetts, not England), here we come! Last but not least, I would like to thank my family—my parents and my sister—for their unconditional love and support in everything I do. Thank you for encouraging me to follow my dreams and for teaching me to value knowledge, education, and hard work. And to never lose hope. vi ABSTRACT This thesis presents theoretical investigations in three areas of astrophysics, all related to radiative processes and interactions between stellar radiation and gaseous media in the Universe, ranging from the intergalactic and interstellar medium to planetary atmospheres. Part I of the thesis consists of two independent investigations in which we study the effects of stellar feedback in high-redshift environments. The topic of Chapter 2 is the intergalactic medium (IGM) in the epoch just after the formation of the first stars in the Universe, but before the cosmic reionization was completed. This epoch is of great interest for the ongoing and future experiments aimed at observing the neutral IGM via the redshifted 21 cm line of hydrogen. We study the effects of resonant scattering of Lyman-α photons produced by early stars on the structure of temperature fluctuations in the IGM. In Chapter 3, we use cosmological hydrody- namic simulations of galaxy evolution to study the effects of stellar feedback on the clumpy structure of star-forming galaxies at z ∼ 2. Observations of high-redshift galaxies show that their morphology is often dominated by a few giant clumps of intense star formation, but the nature and the importance of these clumps for the evolution of their host galaxies are uncertain. We present a detailed analysis of the properties of giant clumps in a high-redshift simulated galaxy from the FIRE project. Part II of the thesis is devoted to the effects of Raman scattering of stellar radiation in the atmospheres of extrasolar planets. Spectral signatures of Raman scattering imprinted in the geometric albedo spectrum of a gaseous planet carry information about the properties of the planet’s atmosphere—its composition, temperature, and the radiation-penetration depth. In Chapter 5, we present the results of radiative transfer calculations including the treatment of Raman scattering for different types of planetary atmospheres and analyze the feasibility of detecting the spectral signa- tures of Raman scattering in nearby exoplanets. The structure and the intensity of Raman spectral features depends on both the atmospheric properties and the shape of the stellar spectrum irradiating the atmosphere. In Chapter 6, we analyze the diversity of Raman features in the geometric albedo spectra of planets hosted by different types of stars. vii PUBLISHED CONTENT AND CONTRIBUTIONS Oklopčić, A., Hopkins, P. F., Feldmann, R., et al., 2017, "Giant clumps in the FIRE simulations: a case study of a massive high-redshift galaxy"; Monthly Notices of the Royal Astronomical Society, 465, 952 doi:10.1093/mnras/stw2754 A.O. co-developed the idea for the project, performed the analysis, and wrote the paper. Oklopčić, A., Hirata, C. M., & Heng, K., 2016, "Raman Scattering by Molecular Hydrogen and Nitrogen in Exoplanetary Atmo- spheres"; The Astrophysical Journal, 832, 30 doi:10.3847/0004-637X/832/1/30 A.O. developed the idea for this project, performed the analysis, and wrote the paper. Oklopčić, A., & Hirata, C. M., 2013, "Lyα Heating of Inhomogeneous High-redshift Intergalactic Medium"; The As- trophysical Journal, 779, 146 doi:10.1088/0004-637X/779/2/146 A.O. performed the analysis for this project and wrote the paper. viii TABLE OF CONTENTS Acknowledgements . iv Abstract . vi Published Content and Contributions . vii Table of Contents . viii List of Illustrations . xi List of Tables . xxiii I Stellar Feedback Effects in the Intergalactic and Interstel- lar Medium at High Redshift 1 Chapter I: Introduction to Part I . 2 1.1 Cosmic Dawn . 4 1.2 Cosmic Noon . 5 Chapter II: Lyman-α Heating of Inhomogeneous High-redshift Intergalactic Medium . 9 2.1 Introduction . 10 2.2 Formalism . 13 2.2.1 Radiative Transfer . 14 2.2.2 Unperturbed Background Solution . 18 2.2.3 Perturbations . 20 2.3 Heating Rates . 20 2.4 Results and Discussion . 25 2.4.1 Heating from Unperturbed Radiation . 25 2.4.2 Heating from Perturbations . 26 2.5 Conclusions . 28 Appendices . 30 2.A System of Equations . 30 2.B Numerical Calculations . 30 Chapter III: Giant Clumps in the FIRE Simulations: a Case Study of a Massive High-redshift Galaxy . 34 3.1 Introduction . 35 3.2 The Simulation . 39 3.2.1 The FIRE project . 39 3.2.2 The m13 simulation . 40 3.3 Clump Identification . 42 3.4 Results . 45 3.4.1 Clump properties in the m13 simulation . 45 3.4.2 Occurrence of clumps over redshifts . 46 3.4.3 Stellar age in clumps . 48 ix 3.4.4 Radial gradients in clump properties . 50 3.4.5 Clump lifetime . 52 3.5 Discussion . 54 3.5.1 Clump formation and destruction . 54 3.5.2 Are clumps gravitationally bound? . 59 3.5.3 Clumps in comparison with GMCs . 64 3.6 Conclusions . 67 Appendices . 69 3.A Clump Properties in Additional FIRE Simulations of Massive Galax- ies at Redshift 2 . 69 3.B Sensitivity to the Clump Finder Parameters . 71 II Raman Scattering in Exoplanetary Atmospheres 78 Chapter IV: Introduction to Part II . 79 4.1 Studies of Exoplanet Atmospheres . 80 4.1.1 Observational techniques . 80 4.1.2 Prevalence of Exoplanet Atmospheres . 82 4.1.3 Compositions . 83 4.1.4 Clouds and Hazes . 84 4.2 Goals of Part II of this Thesis . 85 Chapter V: Raman scattering by molecular hydrogen and nitrogen in exoplan- etary atmospheres . 90 5.1 Introduction . 91 5.2 Raman Scattering . 93 5.3 Methods . 97 5.3.1 Model Atmospheres .