Clustering of Stars in Nearby Galaxies: Probing the Range of Stellar Structures by Catherine Kaleida A Dissertation Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy Approved August 2011 by the Graduate Supervisory Committee: Paul Scowen, Co-Chair Rogier Windhorst, Co-Chair Rolf Jansen Francis Timmes Evan Scannapieco ARIZONA STATE UNIVERSITY December 2011 ABSTRACT Most stars form in groups, and these clusters are themselves nestled within larger associations and stellar complexes. It is not yet clear, however, whether stars cluster on preferred size scales within galaxies, or if stellar groupings have a con- tinuous size distribution. I have developed two methods to select stellar groupings across a wide range of size-scales in order to assess trends in the size distribu- tion and other basic properties of stellar groupings. The first method uses visual inspection of color-magnitude and color-color diagrams of clustered stars to assess whether the compact sources within the potential association are coeval, and thus likely to be born from the same parent molecular cloud. This method was developed using the stellar associations in the M51/NGC 5195 interacting galaxy system. This process is highly effective at selecting single-aged stellar associations, but in order to assess properties of stellar clustering in a larger sample of nearby galaxies, an automated method for selecting stellar groupings is needed. I have developed an automated stellar grouping selection method that is sensitive to stellar clustering on all size scales. Using the Source Extractor soft- ware package on Gaussian-blurred images of NGC4214, and the annular surface brightness to determine the characteristic size of each cluster/association, I elimi- nate much of the size and density biases intrinsic to other methods. This automated method was tested in the nearby dwarf irregular galaxy NGC4214, and can detect stellar groupings with sizes ranging from compact clusters to stellar complexes. In future work, the automatic selection method developed in this dissertation will be used to identify stellar groupings in a set of nearby galaxies to determine if the size scales for stellar clustering are uniform in the nearby universe or if it is dependent on local galactic environment. Once the stellar clusters and associa- tions have been identified and age-dated, this information can be used to deduce disruption times from the age distribution as a function of the position of the stellar i grouping within the galaxy, the size of the cluster or association, and the morpho- logical type of the galaxy. The implications of these results for galaxy formation and evolution are discussed. ii To my family, who encouraged me to reach for the stars... iii ACKNOWLEDGMENTS This dissertation would never have come to fruition without the encourage- ment and shared knowledge of many teachers, friends, and colleagues. My un- dergraduate astronomy professor, Dr. Loris Magnani, once said that anyone can get a Ph.D, if they are willing to put themselves through the pain. I can now fully appreciate the level of commitment he was speaking of, the extreme dedication that every doctoral student must have for their work. I would like to thank Dr. Magnani, and the many teachers who made me believe that I could be one of those people, and for imparting the knowledge necessary to accomplish this task. I would like to express my gratitude to my thesis advisors for their continued advice, insight, and enlightening discussions: Dr. Paul Scowen, for be- ing incredibly supportive and patient at the times that I most needed it and least deserved it, and Dr. Rogier Windhorst, for opening new doors for me, and for al- ways going to bat for his students. I would also like to thank Dr. Rolf Jansen for advising me on my secondary project, and teaching me to how to observe at a research-class telescope. Furthermore, I would like to express my gratitude to Drs. Bradley Whitmore and Rupali Chandar for many invaluable scientific discus- sions, for their patience, for providing guidance, and for teaching me how to ask the interesting questions. I also thank my whole dissertation committee for their feed- back throughout this process. They are Drs. Paul Scowen, Rogier Windhorst, Rolf Jansen, Francis Timmes, and Evan Scannapieco. I am also indebted to Dr. Seth Cohen, for teaching me many, many practical and useful skills that I use every day in my research. Furthermore, I am grateful to my fellow ASU graduate students who have enriched my life, including but not limited to: Hwihyun Kim, for being my friend, collaborator, and mentor, without whom I would be lost; my roommates over the iv years: Angel Fuentes, Dr. Rosa Diaz, Emily McLinden, Matt Metchley, and Dr. Pascale Hibon, for keeping me company and providing an understanding ear; my office-mate and friend, Todd Veach, for his computer expertise and witty sense of humor; and also Dr. Russell Ryan, Dr. Nimish Hathi, Dr. Amber Straughn, Dr. Cynthia D’Angelo,Dr. Michael Lesniak, Brian Gleim, Michael Rutkowski, Dr. Carola Ellinger, and Dr. Vithal Shet-Tilvi. In addition to those already mentioned, I would like to express my thanks to Dr. Bruce Balick for improving my knowledge of the interplay between gas and stars and teaching me how to look at the “big picture”, Zolt Levay for producing the color image of NGC4214 for Figure 3.1, and Dr. Max Mutchler for drizzling the HST WFC3 Early Release Science images of NGC 4214. Special thanks to the people in the SESE and Physics Department administration who have helped me immensely throughout my graduate school career, including: Sunny Thompson, Scott Smas, Teresa Robinette, Becca Dial, and Becky Polley. This work has been supported by programs HST-AR 1068401A, HST- AR 12137, and HST-AR 11360, provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Analysis of NGC 4214, M83, and NGC 2841 was based on Early Release Science observations made by the WFC3 Scientific Oversight Committee. I am grateful to the Director of the Space Telescope Science Institute for awarding Director’s Discretionary time for this program. Last but not least, thank you to my family: Mom, Dad, Beth, and David, and to my dear friends Ariela Friedman and Natalie Beckman. Your unending patience and support have carried me along the way! Final thanks to everyone who has helped me to this point. With the proper encouragement, support, and stubbornness, ordinary people can accomplish extraordinary things. v TABLE OF CONTENTS Page TABLEOFCONTENTS ............................. vi LISTOFTABLES ................................ vii LISTOFFIGURES ................................ viii CHAPTER .................................... 1 1 INTRODUCTION ............................... 1 2 SINGLE-AGED STELLAR ASSOCIATION SELECTION IN THE M51/NGC 5195SYSTEM ................................ 7 2.1 GalaxyOverview ............................ 7 2.1.1M51Dynamics .......................... 7 2.1.2StarClustersandAssociationsinM51. 8 2.2 Observations .............................. 10 2.2.1Pointings ............................. 11 2.3 DataReduction ............................. 11 2.3.1Processing ............................ 11 2.3.2 Photometry and Initial Stellar Association Selection . ... 12 2.3.3 Extinction/Reddening and Redshift Corrections . 13 2.3.4 Stellar Association Selection and Age Determination . .. 15 2.3.5 Other Parameters (Stellar Association Size, Position) . .... 20 2.4 Results ................................. 20 2.4.1 Properties of Isolated Stellar Associations in M51 . 20 2.5 Discussion................................ 21 2.5.1 Age, Size, Mass, and Dust Distribution with Position . 21 2.5.2TheRecentStarFormationHistoryofM51 . 24 2.5.3NextStepsintheAnalysisoftheSystem . 25 2.6 Conclusions............................... 26 vi Chapter Page 3 AUTOMATED STAR CLUSTER/ASSOCIATION SELECTION IN NGC 4214 39 3.1 Overview ................................ 39 3.2 ObservationsandDataProcessing . 41 3.3 SelectionandCatalogsofStellarGroupings. ... 42 3.3.1ManualCatalog . 43 3.3.2AutomaticCatalog . 44 3.3.2.1 Construction of Compact Cluster Catalog using DAOphot 44 3.3.2.2 Construction of Diffuse Association Catalog using Source Extractor ....................... 45 3.3.3 Comparison Between Manual and Automated Catalogs . 47 3.4 SizeMeasurementsandStellarGroupings . 47 3.5 Photometry and Age Estimates of Stellar Groupings . ... 50 3.6 Results and Discussion . 53 3.6.1TheSizeDistributionofStellarGroupings . 53 3.6.2 The Age Distribution of Stellar Groupings in NGC 4214 . 54 3.7 Conclusions............................... 56 4 FINALCONCLUSIONS ........................... 80 4.1 Summary ................................ 80 4.2 FutureWork............................... 83 REFERENCES .................................. 87 vii LIST OF TABLES Table Page 2.1 Stellar Association Parameters for 120 Single-Aged Associations in the M51/NGC5195System........................... 32 3.1 SExtractor Parameter Values Used for each Gaussian Blur Size . ... 72 3.2 MasterTableofStellarGroupingsinNGC4214 . ... 73 viii LIST OF FIGURES Figure Page 2.1 CM and CC diagrams, and V -band images for three example stellar associations from our set. The blue-purple dotted lines are Padova stel- lar isochrones for stellar masses from 0.15 to 66.56 M⊙ , metallicity Z=0.019, and ages are 1 Myr (lightest blue), 6 Myr, 10 Myr, 20 Myr, and