QUANTITATIVE ANALYSIS of 3-ARM SPIRAL GALAXIES By
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QUANTITATIVE ANALYSIS OF 3-ARM SPIRAL GALAXIES by COLIN HANCOCK WILLIAM KEEL PREETHI NAIR JEREMY BAILIN BROOKE SIMMONS A THESIS Submitted in partial fulfillment of the requirements for the degree of Master of Science in the Department of Physics and Astronomy in the Graduate School of The University of Alabama TUSCALOOSA, ALABAMA 2019 Copyright Colin Hancock 2019 ALL RIGHTS RESERVED ABSTRACT A relatively small fraction of spiral galaxies has three spiral arms. It has been theorized that these \3-arm spirals" are relatively unstable and prone to decaying into even numbered patterns due to tidal interactions. We present a series of quantitative analyses on a large sample of 3-arm spiral galaxies selected by the Galaxy Zoo 2 group. Much of this analysis used the image processing interface known as the Spiral Arc Finder and Reporter (SpArcFiRe). This program traced spiral arms on submitted images of galaxies and provided information that allowed us to replicate the arms in another pro- gram like MS Excel. Most of our work involved images taken from the Sloan Digital Sky Survey (SDSS), supplemented with high-resolution Hubble Space Telescope (HST) ob- servations. This allowed us to study the morphological demographics of our sample, for both the internal structure and arm symmetry. The HST images provided special insight into spiral structure and star formation for these galaxies. We analyzed the star forma- tion in particular using the MIRA AL software for photometry. We also attempted in improve the signal-to-noise of our images using data from the Stripe 82 region of SDSS. Unexpectedly, 3-arm patterns coexist with strong central bars roughly as often as other spiral patterns. We also found that 3-arm spirals do not preferentially exist in low-density regions and may be triggered by interactions. ii DEDICATION For my family who kept me grounded, and my friends who kept me sane. iii LIST OF ABBREVIATIONS AND SYMBOLS ACS Advenceed Camera for Surveys DECaLS Dark Energy Camera Legacy Survey DR7/12 Data Release 7/12 ESA European Space Agency FITS Flexible Image Transport System GZ2 Galaxy Zoo 2 HST Hubble Space Telescope IC Index Catalogue NASA National Aeronautics and Space Administration NGC New General Catalogue PC1 Planetary Camera 1 SDSS Sloan Digital Sky Survey SFR Star Forming Region SpArcFiRe Spiral Arc Finder and Reporter SSPSF Stochastic Self-Propagating Star Formation WFPC2 The Wide Field Planetary Camera 2 iv ACKNOWLEDGMENTS Firstly, I would like to express my gratitude to my advisor Dr. Bill Keel for his endless support of my research, as well as the myriad advice and discussions throughout my time here. I would also like to thank the rest of my thesis committee: Dr. Jeremy Bailin, Dr. Preethi Nair, and Dr. Brooke Simmons. Especially Drs. Bailin and Nair, for their teaching and advising over the course of my graduate school career. I extend my thanks to Drs. Bruce and Debbie Elmegreen for their contributions, both for their work before our research and their input in the midst of it. I am also grateful to the creators and participants of the Galaxy Zoo 2 project, without whom none of this would have been possible. I would also like to thank Dr. Amy Jones, for taking the time to proofread this work (among other things). v CONTENTS ABSTRACT.............................................................. ii DEDICATION............................................................ iii LIST OF ABBREVIATIONS AND SYMBOLS................................ iv ACKNOWLEDGMENTS...................................................v LIST OF TABLES......................................................... vii LIST OF FIGURES........................................................ viii 1 INTRODUCTION......................................................1 2 METHODS............................................................4 3 ANALYSIS............................................................. 12 3.1 Morphological Types . 12 3.2 SpArcFiRe and Excel . 14 3.3 HST Data . 17 3.4 Stripe 82 . 23 4 CONCLUSION......................................................... 26 5 REFERENCES......................................................... 46 vi LIST OF TABLES 2.1 Table explaining each step in Figure 2.6. 11 3.1 Sorting of galaxies according to internal morphology and arm symmetry, with corresponding percentages across from each label. 12 3.2 Data for each HST image from Figure 3, primarily from the FITS Headers. 18 3.3 Coordinates and brightness of each SFR labelled in Figure 3.7. 23 4.1 Candidate 3-arm spirals examined. 29 vii LIST OF FIGURES 1.1 A comparison of a grand design galaxy with a flocculent spiral galaxy . 2 2.1 Decision tree for Galaxy Zoo 2, taken from Willett et al. [20]. 5 2.2 Histogram showing the distribution of neighbor density for galaxies from the 3-arm GZ2 sample. 6 2.3 Histogram showing the distribution of redshifts for galaxies from the 3-arm GZ2 sample. 7 2.4 Chart displaying the pitch angles taken from Kennicutt [14] alongside the cor- responding average angles reported by SpArcFiRe for the 40 galaxies that were observed by SDSS. 8 2.5 Chart describing the correlation of pitch angles described by Kennicutt [14] with the average angles reported by SpArcFiRe for the 40 galaxies that were observed by SDSS. 9 2.6 Example of the processing undertaken by SpArcFiRe. 10 3.1 A visual display of the galaxies from our sample sorted according to internal morphology and arm symmetry. 13 3.2 Example of galaxy arcs from SpArcFiRe replicated in MS Excel. 16 3.3 Example of a galaxy whose arms have very little overlap in radius. 17 3.4 Cropped images of the two spiral galaxies that we obtained HST data for. 18 3.5 Comparison of SDSS 1704 images taken with SDSS and HST. 19 3.6 Zoomed-out and rotated image of SDSS 1704 with an interacting companion. 20 3.7 Side-by-side comparison of IC 3528 with and without photometric regions. 22 viii 3.8 Side-by-side comparison of SpArcFiRe analysis from SDSS DR12 and Stripe 82......................................... 25 4.1 Montage of 3-arm spiral candidates paired with their corresponding final SpAr- cFiRe outputs, showing the overlaid logarithmic spiral arcs. 34 4.2 Montage of galaxies analyzed by SpArcFiRe, as described by Figure 4.1 (con- tinued). 35 4.3 Montage of galaxies analyzed by SpArcFiRe, as described by Figure 4.1 (con- tinued). 36 4.4 Montage of galaxies analyzed by SpArcFiRe, as described by Figure 4.1 (con- tinued). 37 4.5 Montage of galaxies analyzed by SpArcFiRe, as described by Figure 4.1 (con- tinued). 38 4.6 Montage of galaxies analyzed by SpArcFiRe, as described by Figure 4.1 (con- tinued). 39 4.7 Montage of galaxies analyzed by SpArcFiRe, as described by Figure 4.1 (con- tinued). 40 4.8 Montage of galaxies analyzed by SpArcFiRe, as described by Figure 4.1 (con- tinued). 41 4.9 Montage of galaxies analyzed by SpArcFiRe, as described by Figure 4.1 (con- tinued). 42 4.10 Montage of galaxies analyzed by SpArcFiRe, as described by Figure 4.1 (con- tinued). 43 4.11 Montage of galaxies analyzed by SpArcFiRe, as described by Figure 4.1 (con- tinued). 44 4.12 Montage of galaxies analyzed by SpArcFiRe, as described by Figure 4.1 (con- tinued). 45 ix 1 INTRODUCTION A large fraction of galaxies in our local universe are what we refer to as \Spirals." These possess features such as a flat disk profile, a dense central bulge, and the epony- mous spiral arms that extend from the center. In addition, they may also have a bar- shaped mass distribution across their center, as well as diffuse spherical halos containing star clusters and dark matter. Ever since their spiral nature was first observed in the 1800's by William Parsons, 3rd Earl of Rosse, they have been captivating astronomers with their vibrant colors and elegant structure. While they are some of the most visu- ally striking objects in the universe, it is still somewhat unclear how exactly the spiral structure is formed. One of the fundamental problems of spiral arms is why they do not \wind up." That is, given that these galaxies are differential rotators, their arms should wind more and more tightly after only a few galactic rotations. The universe is roughly 14 billion years old, while galactic rotation periods are on the order of several hundred million years. Since we still see plenty of spirals, there must be some mechanism stabilizing the spiral arms (D'Onghia [10],Masters et al. [17]). The two prevailing (but not mutually exclusive) theories regarding this are: the density wave theory and the stochastic self- propagating star formation (SSPSF) model. Under density wave theory (or Lin-Shu theory after Lin & Shu [15]), the spiral arms are comprised of a long-lasting and self-propagating density pattern with a particu- lar speed that is independent of the stars and gas in the disk. This results in the leading edge of the density waves compressing the matter as they pass through, causing a wave of star formation as well as enhancing its own density. By contrast, in the SSPSF model from Seiden & Gerola [19] shock waves from supernovae perturb the interstellar medium, 1 triggering more star formation. With the differential motion of stars in the disk this cre- ates the spiral pattern. While both theories are valid, SSPSF is more generally applica- ble (and works for irregular galaxies as well). Conversely, density wave theory is strictly relevant to spirals. These two methods of arm formation also apply to the different variations of spi- ral arms. Two broad categories of spirals are known as \grand design" and “flocculent." Grand design spirals possess well-defined and easily distinguishable arms, while floccu- lent spirals are “fluffy” with arms that are broken-up and fuzzy (defined in optical). A comparison of the two types can be found in Figure 1.1. Broadly speaking, density wave theory is better for explaining grand design galaxies, while SSPSF is better for floccu- lent spirals.