ALMOST DARK GALAXIES IN THE ALFALFA SURVEY 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 Lukas Ross Leisman August 2017 c 2017 Lukas Ross Leisman ALL RIGHTS RESERVED ALMOST DARK GALAXIES IN THE ALFALFA SURVEY Lukas Ross Leisman, Ph.D. Cornell University 2017 Star formation laws in galaxies appear to be tied to their available reservoirs of neutral hydrogen (H I). However, gas rich sources that are unable to convert their H I into stars would be difficult to detect in optical surveys, potentially dipping below detectable surface brightness limits in the most extreme cases. Theoretical predictions have been mixed as to whether these potential “(almost) dark,” H I rich sources should exist. Thus, one of the main scientific drivers behind blind H I surveys is the search for optically dark, or almost dark, but gas-bearing dark matter halos. The Arecibo Legacy Fast ALFA blind H I survey (ALFALFA) has detected over 30,000 clearly extragalactic sources, ∼99% of which can be readily identi- fied with probable stellar counterparts in public SDSS or DSS2 images. Here we present the results of follow up efforts to understand the other ∼300 “dark” systems in ALFALFA, in a search for potential “almost dark” galaxies. We expect most optically “dark” ALFALFA sources to be H I clouds stripped from galaxies in tidal interactions: tidal interactions are well known to play an important role in galactic evolution in group environments. Indeed, we confirm that over half of the “dark” systems in ALFALFA have a nearby neighbor (pro- jected within 100 kpc and at a similar redshift) or are a clear tidal appendage. However, in at least two cases we found that candidate “dark” sources far from their nearest neighbor, were in fact part of large H I plumes with extents exceed- ing 1.4 deg (∼600 kpc). These “dark” H I plumes constitute up to 40% of the total H I mass in their host groups, potentially implying significant undetected H I in other groups, and greater prevalence of group scale interactions than pre- viously thought. These observations demonstrate the importance of sensitive, wide field H I surveys in understanding the role of groups in galaxy evolution. After removing tidal plumes, searching for (almost) dark galaxies requires extensive confirmation observations to remove spurious sources and OH Mega- masers (the 18 cm line from rare molecular masers in luminous infrared galaxies redshifted into the ALFALFA bandpass). Further, since Arecibo does not resolve sources with its ∼3.50 beam, determining their nature requires detailed optical and H I follow up observations. Through a series of H I observations with the Jansky Very Large Array (VLA) and the Westerbork Synthesis Radio Telescope (WSRT), and optical observations with the WIYN 3.5m Telescope at Kitt Peak National Observatory, we have imaged 25 of the best ALFALFA (almost) dark candidates. One early result of this imaging was the discovery of AGC 229385, a bright H I source (log (MHI/M ) = 8.7 at D=25 Mpc), but dark at the surface brightness limit of SDSS. Deep imaging revealed a very blue optical counterpart, with a 2 peak surface brightness of µg = 26.4 mag/arcsec , an order of magnitude fainter than low surface brightness (LSB) galaxies detected in SDSS, placing it far off H I-stellar mass scaling relations. AGC 229385 has the largest accurately mea- sured H I mass to light ratio of any known non-tidal, optically detected galaxy, MHI/LB = 38 M /L , and appears to lie far off the Tully-Fisher relation. The H I and optical follow up observations also reveal that several of the “al- most dark” sources are undetectable in current optical surveys not because they do not have stars, but because the stars are too spread out. These sources ap- pear to be “ultra diffuse” galaxies (UDGs) - extreme galaxies with stellar masses of dwarf galaxies, but radii similar to galaxies as large as the Milky Way. Most previously detected UDGs are in clusters, without detected H I. We broaden our definition of “almost dark” to search for more UDGs in ALFALFA, and present a sample of ∼100 isolated ALFALFA galaxies that have similar stellar surface brightnesses, radii, and absolute magnitudes to optically selected UDGs in denser environments. Though these ALFALFA “H I-bearing ultra-diffuse” galaxies have similarly large radii for their stellar mass, they differ from clus- ter UDGs in that they are richer in atomic gas, bluer, and have more irregular morphologies. We present follow up H I and optical imaging of three of these H I-bearing UDGs, and show significant diffuse optical emission, large H I disks extending well beyond the stellar emission to radii of nearly 20 kpc, and evi- dence of ordered rotation. Indeed, comparison of this rotation to halo models may suggest that these sources have halo masses more consistent with dwarf galaxies, and may suggest they formed in high angular momentum halos. BIOGRAPHICAL SKETCH Lukas (Luke) Leisman grew up in Grand Rapids, Michigan as the oldest of 4. He received his Bachelors of Science in Physics from Calvin College in 2011, and Masters of Science in Astronomy from Cornell University in 2014. As a graduate student at Cornell, he spent most of his time in the “Camuy Cave” as a member of the extragalactic group (EGG) and the ALFALFA team, studying the evolution and origin of galaxies outside of the Milky Way using data from the Arecibo radio telescope in Puerto Rico. He also spent much of his time teaching, including involvement with the Cornell Prison Education Program. When he is not studying astronomy, he spends time with friends, contemplating the meaning of the universe, and exploring gorgeous Ithaca, NY. He also enjoys reading, writing, violin, sports, and understanding the relationship between his science and his Christian faith. He will be continuing his work teaching and studying galaxies as an Assistant Professor of Physics and Astronomy at Valparaiso University starting in the fall of 2017. iii For my great cloud of witnesses: Katelyn, my family, and everyone else who kept me sane along the way. iv ACKNOWLEDGEMENTS It is impossible to adequately thank the many family, friends, teachers, and co- workers who have made this thesis possible. I was privileged to have the op- portunity to even begin this journey, and have been blessed with a network of supportive individuals who have made this experience a success. To all those who have encouraged and supported me along the way, thank you. Though this list is woefully incomplete, I would like to specifically acknowl- edge a number of individuals who have made invaluable contributions to this work: Katelyn Leisman – Katelyn is simply the best. From LAT X help to keeping • E me on task to her constant encouragement, this thesis is what it is thanks to her. Mom, Dad, Hannah, Dorthea, and Hans – It is often said that people make • sense in the context of their family. So much of who I am is shaped by their influence, love, and support. I succeed because of my family. Martha Haynes – One of the pieces of advice I received most often as a • young grad student was that one’s advisor makes or breaks one’s grad- uate school experience, so choose wisely. I must say that Martha made my graduate school experience. She was an expert teacher, orchestrator, guide, and advocate. Martha is a skilled mentor who cares about her stu- dents – I very much appreciated her ability to guide and aid my thesis while still giving me the freedom to choose my own way, enabling and encouraging my growth as a scientist. I am lucky to have had her as an advisor. Riccardo Giovanelli – As my second advisor, Riccardo also helped make • my graduate school experience. His thoughtful words of wisdom have v guided my approach to problem solving, and his incisive questions have been instrumental in the success of this work. More, his dry sense of humor and love of football (soccer) were essential components to every group meeting. It is also important to know that Riccardo is also the orig- inal discover of HI1232+20 (chapter 4), which we call “Coma P” after him, and deserves extensive credit for making the ALFALFA dataset a reality through his tireless hours of source extraction. Mike Jones – Mike and I joined the ALFALFA team at essentially the same • time, which was lucky for me. Mike was a constant and patient sound- ing board for my crazy, stupid, and other ideas, and was an invaluable resource for working through problems. Mike was also an excellent office mate, and a good friend, and I will miss his excellent puns, British accent, and steady encouragement. Greg Hallenbeck – Greg has been an excellent mentor, and a great friend. • He is the person most responsible for teaching me the ins and outs of H I data analysis, and has shared much code and documentation that were the produces of months and years of work. More, he opened his home to Katelyn and me on short notice, and has been our companion for innu- merable board games, library book sales, discussions of the book of Enoch, and much more. Becky Koopmann and the undergraduate ALFALFA team –Becky and the • other members of the undergraduate ALFALFA team – students and fac- ulty – have been instrumental in my development as a teacher and as a scientist. I have learned so much from them. More, they have contributed significant effort to the analysis of data in this thesis, especially the under- graduate students David Bernal, Tayeb Zaidi, Catie Ball, Quinton Singer, vi and Charlotte Martinkus to whom I own much thanks.