EXPLORING THE ENVIRONMENTS OF LONG-DURATION GAMMA-RAY BURSTS A DISSERTATION SUBMITTED TO THE GRADUATE DIVISION OF THE UNIVERSITY OF HAWAI`I IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN ASTRONOMY August 2010 By Emily M. Levesque Dissertation Committee: L. J. Kewley, Chairperson A. Boesgaard F. Bresolin R. Kudritzki J. Learned P. Massey We certify that we have read this dissertation and that, in our opinion, it is satisfactory in scope and quality as a dissertation for the degree of Doctor of Philosophy in Astronomy. DISSERTATION COMMITTEE Chairperson ii c Copyright 2010 by Emily M. Levesque All Rights Reserved iii For Heather, who showed me what it means to reach for the stars. And for Pepere, who always believed I was already there. iv Acknowledgements First and foremost, thanks so much to my enthusiastic, tireless, and talented adviser, Lisa Kewley, for being an inspiring teacher, guide, and role model throughout the course of my time here as a grad student. Warm thanks to the rest of my thesis committee - Ann Boesgaard, Fabio Bresolin, Rolf Kudritzki, John Learned, and Phil Massey - for their valuable and thought-provoking input on my increasingly ungainly thesis. A special acknowledgment goes to Phil for his sanity-saving guidance, advice, and friendship throughout all of my astronomy adventures! Much gratitude must also go to Edo Berger at Harvard University's Center for Astrophysics for his guidance and assistance in deciphering the gritty details of every GRB host I had to wrestle with. Thanks as well to Alicia Soderberg, Bob Kirshner, and Scott Kenyon for their collaboration, advice, and support during my time as a CfA predoc. I am indebted to the wonderful support staff, astronomers, and telescope operators of the Keck Observatories and Las Campanas Observatory, in particular: Greg Wirth, for his invaluable guidance on observing with LRIS; Scott Dahm and Jim Lyke, for their vital assistance with NIRSPEC; and Nidia Morrell, for her mission-critical advice on using IMACS. Thanks to my many collaborators in our work on GRB and SN host galaxies (Megan Bagley, Josh Bloom, Nat Butler, Brad Cenko, Lisa Chien, Ryan Chornock, Andy Fruchter, John Graham, Emeric Le Floc'h, Maryam Modjaz, Dan Perley, Jason Prochaska, Sandra Savaglio, Christina Th¨one,and Tiantian Yuan), stellar population synthesis and photoionization modeling (Kirsten Larson, Claus Leitherer, Daniel Schaerer, and Leonie v Snijders), and red supergiants (Phil Bennett, Geoff Clayton, Peter Conti, Eric Josselin, Andre Maeder, Georges Meynet, Knut Olsen, Bertrand Plez, and Dave Silva). I wouldn't have published a word, traveled anywhere, or gotten paid without the unfailing patience and administrative magic of Amy Miyashiro. Thanks as well to Josh Barnes, Lisa Catella, Christine Crowley, Shadia Habbal, Nancy Lyttle, Dave Sanders, Lori Serikawa, Pui-Hin Rhoads, Narayan Raja, Sue Tedeski-Hamelin, Karen Teramura, Diane Tokumura, Bill Unruh, and Karl Uyehara, without whom I would have none of the critical paperwork, no working projectors, no internet access, no data analysis software, no room reservations, and no keys to the building! I am also grateful for the support I received from the Ford Predoctoral Diversity Fellowship and the Smithsonian Astrophysical Observatory Predoctoral program. My fellow Institute for Astronomy grad students, past and present, have been an endless font of research help, code triage, advice, encouragement, and therapy during the past four years! Special thanks to the class of \zzzz"s - Cooper, Geoff, Rach, Sonnett, Vivian, and Tiantian - and to the past and present residents of Grad House for their generosity with futons, wine, cheese, camaraderie, and the Wii. Thanks as well to the faculty and staff at the IfA for making the department such an immensely enjoyable place to work, learn, and grow as a scientist. A lifetime of thanks must go to Dad, for dragging the 8" Celestron out of the basement on lots of cold Massachusetts nights; to Mom, for doggedly insisting to the powers-that- be that yes, there was in fact \math after Calculus"; and to my brother, for perpetually giving me the dauntingly high goal of \I wanna be like Ben!" to strive for. Deep love and appreciation to Andrea, Jocelyn, Aaron, and every single member of the big crazy Levesque and Cabana families for their unwavering support, and to Sarah, Meredith, Cam, Heather, Jake, Glen, Gina, Lisa, and the rest of the Pi Tau Zeta, MIT, Boston, and Colorado folks for the adventures and friendship. Finally, unending love and bottomless gratitude to Dave for his encouragement, patience, and willingness to put himself at the front lines of a chaotic Ph.D. thesis for four long, vi challenging, and wonderful years. I love you as big as the sky...and coming from an astronomer, that is truly saying something. vii Abstract Long-duration gamma-ray bursts (LGRBs) are the signatures of extremely energetic phenomena occurring throughout our universe. These events, commonly thought to be associated with the deaths of massive stars, have been proposed as possible tracers of star formation at high redshift; however, such an association is dependent on a thorough understanding of LGRB host environments and progenitors. In particular, the metallicity of LGRB host galaxies has become a matter of hot debate in recent years, with several studies suggesting that these events may be biased towards low-metallicity environments. The main goal of this dissertation is to perform the first in-depth study of the ISM environments and host galaxies that produce LGRBs. I have conducted the first dedicated spectroscopic survey of LGRB host galaxies, and used these observations along with data from the literature to determine a wide range of ISM properties for 16 z < 1 LGRB hosts and compare them to the general star-forming galaxy population. This work constructs the first mass-metallicity relation determined for LGRBs out to z ∼ 1. I have also generated an extensive suite of new stellar population synthesis and photoionization models, tailored towards modeling the host environments of LGRBs - these models show key improvements over past work, but also highlight several shortcomings in current model codes that must be addressed in future studies. Finally, I have examined red supergiants in low-metallicity Local Group galaxies, a poorly-understood but critical mass-losing phase of massive stellar evolution. From this work, I have concluded that LGRBs do exhibit a trend towards lower-metallicity host environments. However, observations of high-metallicity LGRB host galaxies and a comparison of the energetic and environmental properties of LGRBs both demonstrate that viii the complex role metallicity plays in LGRB progenitor formation remains unclear. New generations of galaxy models and continued studies of massive stellar evolution in low- metallicity environments are both vital to improving our understanding of the progenitors and host galaxies that give rise to these enigmatic events. ix Table of Contents Acknowledgements . v Abstract . viii List of Tables . xiii List of Figures . xiv Chapter 1: Introduction . 1 1.1 Properties of LGRB Host Galaxies; Survey and Key Physical Properties . 6 1.2 Modeling of Star-Forming Galaxies . 10 1.3 Massive Stellar Evolutionary Theory . 12 Chapter 2: LGRB Host Galaxies - Observations and Analyses . 15 2.1 The Nearby LGRB Host Galaxy Survey . 15 2.1.1 Keck: GRBs 980703, 991208, 010921, 020819, 020903, 031203, 030329, 051022, 060218, and 070612A . 15 2.1.2 Magellan: GRB 020405 and GRB 050826 . 18 2.1.3 Published LGRB Host Spectra: GRB 980425, GRB 990712, GRB 030528, and GRB 050824 . 18 2.1.4 Data Reduction . 19 2.2 Analysis of Host ISM Properties . 20 2.2.1 Emission Line Fluxes . 20 2.2.2 Metallicity . 23 2.2.3 Star Formation Rates . 27 x 2.2.4 Young Stellar Population Ages . 27 2.2.5 Stellar Masses . 31 2.2.6 AGN Activity in the Host of GRB 031203 . 31 Chapter 3: LGRB Host Galaxies - Comparison and Interpretation . 33 3.1 LGRB Hosts and the General Galaxy Population . 33 3.1.1 Comparison Samples . 33 3.1.2 Emission Line Ratio Diagnostic Diagrams . 40 3.1.3 ISM Properties . 45 3.2 The Mass-Metallicity Relation for LGRB Hosts . 50 3.3 Host Metallicity and the Isotropic Energy Release of LGRBs . 54 3.4 Discussion . 59 Chapter 4: Unusual Events and Their Host Galaxies . 64 4.1 The High-Metallicity Host of the \Dark" GRB 020819 . 64 4.1.1 Observations . 65 4.1.2 ISM Properties . 66 4.1.3 Discussion . 68 4.2 The Relativistic Supernova 2009bb . 69 4.2.1 Observations . 71 4.2.2 Physical Properties of the SN 2009bb Environment . 71 4.2.3 Comparison with Nearby (z < 0:3) Galaxy Samples . 74 4.2.4 Discussion . 74 4.3 The Environment of the z = 2:609 Short GRB 090426 . 78 4.3.1 Discovery and Afterglow Observations of GRB 090426 . 80 4.3.2 Analysis and Interpretation . 82 4.3.3 The Host Galaxy of GRB 090426 . 83 4.3.4 Discussion . 86 Chapter 5: Stellar Population Synthesis and Photoionization Models - Design and Applications . 88 xi 5.1 Introduction . 88 5.2 Starburst99/Mappings III Model Grids . 92 5.2.1 Model Grid Parameters . 92 5.2.2 Stellar Evolutionary Tracks . 95 5.2.3 Starburst99 Ionizing Spectra . 97 5.3 Optical Emission Line Diagnostics . 102 5.3.1 [NII]/Hα ................................. 104 5.3.2 [NII]/[OII] . 105 5.3.3 [OIII]/Hβ ................................. 106 5.3.4 [OIII]/[OII] . 106 5.3.5 [SII]/Hα .................................. 108 5.4 Emission Line Diagnostic Diagrams . 109 5.4.1 Comparison With Star-Forming Galaxies . 109 5.4.2 Comparison with LGRB Host Galaxies . 118 5.4.3 Comparison with Previous Model Grids . 121 5.5 Late-Age Models and the Stellar Populations of LGRB Hosts .