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Environmental Influences on Dwarf Galaxy Evolution: the Group Environment

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ENVIRONMENTAL INFLUENCES ON DWARF GALAXY EVOLUTION: THE GROUP ENVIRONMENT 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 Sabrina Renee Stierwalt February 2010 c 2010 Sabrina Renee Stierwalt ALL RIGHTS RESERVED ENVIRONMENTAL INFLUENCES ON DWARF GALAXY EVOLUTION: THE GROUP ENVIRONMENT Sabrina Renee Stierwalt, Ph.D. Cornell University 2010 Galaxy groups are a rich source of information concerning galaxy evolution as they represent a fundamental link between individual galaxies and large scale structures. Nearby groups probe the low end of the galaxy mass function for the dwarf systems that constitute the most numerous extragalactic population in the local universe [Karachentsev et al., 2004]. Inspired by recent progress in our understanding of the Local Group, this dissertation addresses how much of this knowledge can be applied to other nearby groups by focusing on the Leo I Group at 11 Mpc. Gas-deficient, early-type dwarfs dominate the Local Group (Mateo [1998]; Belokurov et al. [2007]), but a few faint, HI-bearing dwarfs have been discovered in the outskirts of the Milky Way’s influence (e.g. Leo T; Irwin et al. [2007]). We use the wide areal coverage of the Arecibo Legacy Fast ALFA (ALFALFA) HI survey to search the full extent of Leo I and exploit the survey’s superior sensitivity, spatial and spectral resolution to probe lower HI masses than previous HI surveys. ALFALFA finds in Leo I a significant population of low surface brightness dwarfs missed by optical surveys which suggests similar systems in the Local Group may represent a so far poorly studied population of widely distributed, optically faint yet gas-bearing dwarfs. The morphological segregation seen in the Local Group is also reflected in Leo I and further sug- gests a significant population of gas-bearing dwarfs may be missed by surveys narrowly focused around more massive systems. The Leo I HI mass function is dominated by low mass objects yielding a steeper low-mass slope than found for luminosity functions of the group. However, the slope still falls short of that predicted by simulations of structure formation. Further contributors to this gap may be dwarf systems formed from tidal material (TDGs), the fraction of which, even in the Local Group, remains unknown. We find that TDGs can be identified from the ALFALFA survey based on their proximity to tidal rem- nants and from optical spectroscopic followup via their high gas fractions and high metallicities given their luminosities. However, despite the two large tidal remnants found in Leo I, our search results in only two TDGs for the group. If most dwarfs are instead formed from small dark matter haloes as suggested by the ΛCDM framework, different classes of dwarf may reflect early versus late stages of evolution. Dwarfs of mixed morphologies, like the six so-called tran- sition dwarfs in the Local Group, may represent the evolutionary link between gas-rich and gas-poor classes, but when constrained by the requirement of HII regions, we find only one such candidate in Leo I. The work presented here lays the groundwork for future HI-based group studies which will be made possible with rich ALFALFA dataset. BIOGRAPHICAL SKETCH Sabrina Stierwalt was born in Indiana, spent her early childhood in southern California, and grew up in Norwalk, Connecticut. Before moving to upstate New York to do her PhD work at Cornell, she earned her undergraduate degree in physics and astronomy from the University of California, Berkeley. Ithaca and Berkeley share many of her favorite things, from drum circles and open- minded people to some of the best locally grown, vegetarian food. She will miss walks through the Plantations, the barbeque tofu ’n’ grits at Maxie’s, and the movies at Fall Creek. However, she is happy to be giving up her winter clothes and to no longer have to worry whether or not her car doors will be frozen shut. She begins a postdoctoral position at the California Institute of Technology in August working with Spitzer observations of luminous infrared galaxies. She plans to drive out there with her two good friends, Ann Martin and Betsey Adams, and with her two bulldogs, Bella and Chewie. iii This thesis is dedicated to my brother, Trent. No one else has believed in me so unconditionally as he has. I love you, buddy. iv ACKNOWLEDGEMENTS I just want more than one membership to more than one club and I owe my life to the people that I love. - Ani DiFranco To my collaborators, Ame´lie Saintonge, Becky Koopmann, Andrew West, Sarah Higdon, Ann Martin, Brian Kent, Betsey Adams, John Salzer, Noah Brosch, and Barbara Rojas Ayala: your thoughts, suggestions, and patience were all very much appreciated. To my fellow EGGs, past and present, Ann, Betsey, Ame´lie, Brian, Karen, Kristine, Chris, Kelley, Drew, Manolis, Shan, Nancy, Katie, Kim, Lamarr, and Jae: I have learned so much from all of you. Thank you for making the Camuy Cave such a fun place to work and for taking over the ALFALFA observations throughout the past year so that I could finish my dissertation. To Tom Shannon: Thank you for keeping hatillo alive long enough for me to complete my dissertation and for responding kindly when I called you in a panic on a Saturday night after a power outage. To the telescope operators, Jean, Carl, Norberto, Israel, Willy and Ed: Thank you for your guidance and for humbling me with your far superior knowledge of some of the largest telescopes in the world. To Gary Brinson and the Brinson Foundation: Thank you for promoting my work as a young researcher. Your contributions have allowed me the opportu- nities to travel to conferences to learn from other astronomers and to share my own results. And to my family and friends who supported me in more ways than I could ever ask for v To my mom: you are a caring, patient goddess of a woman who has taught me always to look for the good in people (although, Im sorry, you are much better at it than I am). You never pushed me to study harder or even suggested that I go to grad school, but your example taught me never to underestimate myself. To my brother, Trent: you have such a good heart. Thank you for always supporting me, making me laugh, and for helping me with many of the images that went into this dissertation. Most of all, thank you for being the only person in the whole wide world that understands the madness we call our family. To my sister, Rachel: you showed me that theres no point in watching movies with sad endings and that even though I think I look cool, I totally dress like a scientist. Thank you for allowing my shameless attempts to turn my nieces and nephew into little astronomers. To my father: you passed to me a very valuable skill for an astronomer being able to exist for months at a time with only four hours of sleep per night. Thank you also for helping me to travel the world so that I could see the stars from over two dozen countries. To Kim: you are so clever and you think that I can write a book. If I ever do, it will certainly be inspired by your curiosity as to what I actually do with my time and your endless patience with my long explanations. Thank you for your frequent reminders that astronomy is totally awesome. To Lauren: I cannot believe you trekked all the way to Ithaca to witness my defense. Weve known each other for 15 years, ever since I got caught trying to beat you up during study hall. I love you (and fight with you) like a sister so your support over these last few years has encouraged me to work harder. Alicia: you are the reason I am an astronomer. I followed you into astro lab vi and never looked back. Thank you for being my partner in lab and partner in crime for four years at Berkeley. You make me smarter, and you make my life more interesting. Molly: you are fearless. Thank you for making my grad school years easier by allowing me to rest assured that you would some day have it all figured out and would fix the worlds problems. Nozomi: you are brilliant, fun, and as I see it, amazingly talented at every- thing you try. Your constant encouragement, even when you had your own dissertation to write, has been so important to me these past few years. Thank you for letting me crash your defense so that I could see how it’s done right! To my activist friends, Laura, Liana, Hannah, Claire, Ariela, Erica, Aly, Molly, Jenny, Graham, Julie, Morgan, Leyla, Jaime and Ashwin: Thank you for constantly reminding me that one driven person (or an awesome-sauce group of people!) can make a difference. Also, always be careful of the hole in the ground outside of Pixel! To Randall Pants: Even though you left me to face my last years in Ithaca alone, you always encouraged me when I needed it with your stories and your care packages from Finland. Remember, that inappropriate is in the eye of the beholder, or in our case, in the eye of the one wearing the zebra pants. To Yanling: Despite your complete and utter failure at all attempts to teach me Chinese, I am entirely confident that you are one day going to take over the world, and I cant wait to watch you do it from up close again.
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