Nuclear Outbursts in the Centers of Galaxies A dissertation presented to the faculty of the College of Arts and Sciences of Ohio University In partial fulfillment of the requirements for the degree Doctor of Philosophy Reza Katebi December 2019 © 2019 Reza Katebi. All Rights Reserved. 2 This dissertation titled Nuclear Outbursts in the Centers of Galaxies by REZA KATEBI has been approved for the Department of Physics and Astronomy and the College of Arts and Sciences by Ryan Chornock Assistant Professor of Physics and Astronomy Florenz Plassmann Dean, College of Arts and Sciences 3 Abstract KATEBI, REZA, Ph.D., December 2019, Physics Nuclear Outbursts in the Centers of Galaxies (182 pp.) Director of Dissertation: Ryan Chornock This dissertation consists of two parts. In the first part, we focus on studying the nuclear outbursts in the centers of galaxies and their nature in order to better understand the behavior of central Super Massive Black Holes (SMBHs) and their interaction with the surrounding environment, and to better understand the accretion disk structure. Nuclear outbursts can be better understood by studying the changes in the broad emission lines and the underlying continuum. We quantify the properties of these nuclear outbursts using multi-wavelength observations including optical, ultraviolet, and X-rays from MDM Observatory, the Sloan Digital Sky Survey, Swift, and Magellan. Some of these nuclear outbursts are linked to Tidal Disruption Events (TDEs) and nuclear supernovae (SNs), while a number of these events are proposed to be a rare phenomenon called “changing-look” Active Galactic Nuclei (AGN). These types of AGNs have been observed to optically transition from type 1 to type 2 and vice versa on timescales of months to years, where broad emission lines such as Hα and Hβ appeared or disappeared followed by an increase or decrease in the continuum light. We investigate two transient events called PS1-13cbe and PS1-10cdq that were observed during outburst by the PS1 survey in 2013 and 2010, respectively. We investigate TDE, SN, and AGN activity as the three possible scenarios for the nature of these events. In the case of PS1-13cbe, we conclude that “changing-look” behavior caused by thermal accretion disk instabilities is the most plausible explanation for the outburst. However, in the case of PS1-10cdq, we favor the tidal disruption scenario because of the structure of the lightcurve and spectral evolution. In the second part of this dissertation, we focus on galaxy morphology prediction using a newly designed neural network called “Capsule Networks”. We 4 automate the process of morphology prediction and eliminate the need for feature engineering and heavy data prepossessing prior to classification. We also reconstruct the galaxy images while preserving the brightness structure of the galaxies. This study provides one of the possible solutions for classifying objects for the era of large sky surveys where the amount of data for galaxies will dramatically increase and automated methods will play a very crucial role. 5 Dedication To my father who taught to me to question everything To my mother who has always encouraged me to pursue my dreams To my sisters who have always supported me To my wife Volha who truly inspires me to be the best of myself and To everyone who is brave enough to wander in the realm of unknown and seek answers. 6 Acknowledgments During my PhD, I had the privilege of meeting so many kind and smart people that I want to thank for their help and support. But at first I want to take a moment and express my deepest gratitude toward my advisor Professor Ryan Chornock who taught me the real meaning of science. Whenever I was struggling with a problem he was there to help me to move toward the right direction with his exceptionally vast knowledge. He taught me the correct path of scientific research and made me the physicist I am today. I would like to acknowledge him for his endless support throughout my PhD; no matter where I am or whoever I become, I will always be his lifelong student. I would like to thank physics and astronomy staff members for their effort to provide an efficient and proper atmosphere for research and learning. I would like to express my gratitude toward the members of my dissertation committee Dr. Razvan Bunescu, Dr. Madappa Prakash, and Dr. Joseph Shields for kindly accepting to be involved in my PhD. I would like to specially thank my wife Volha who has always supported me with love and inspired me to be better and stronger. At last but not least, I would like to thank my mother Sakineh, and my sisters Soheyla, Soraya, and Romina for always being there for me and their continuous support. 7 Table of Contents Page Abstract.........................................3 Dedication........................................5 Acknowledgments....................................6 List of Tables...................................... 10 List of Figures...................................... 11 List of Acronyms.................................... 15 List of Sorted Acronyms................................ 17 1 Introduction..................................... 19 1.1 Active Galactic Nuclei............................ 19 1.1.1 AGN Classification.......................... 20 1.1.2 LINERs................................ 24 1.1.3 AGN Unification........................... 25 1.1.4 Accretion Process and Variability in AGNs............. 26 1.2 Changing Look AGNs............................ 30 1.3 Tidal Disruption Events........................... 33 1.4 Supernovae.................................. 38 1.4.1 Superluminous Supernovae...................... 38 1.4.2 Sources that Power Superluminous Supernovae........... 39 1.4.2.1 Circumstellar Interaction................. 39 1.5 The Modular Open Source Fitter for Transients............... 41 1.6 Galaxy Morphology Prediction........................ 42 1.6.1 Deep Learning............................ 44 1.6.1.1 Feed-Forward Neural Networks.............. 44 1.6.1.2 Convolutional Neural Networks.............. 46 2 PS1-13cbe: The Rapid Transition of a Seyfert 2 to a Seyfert 1........... 49 2.1 Introduction.................................. 49 2.2 Observations of PS1-13cbe.......................... 51 2.2.1 Optical Photometry.......................... 52 2.2.2 Observations of the Host Galaxy................... 54 2.2.3 X-ray Photometry........................... 55 2.2.4 Optical Spectroscopy......................... 55 8 2.3 Observational Features of PS1-13cbe..................... 57 2.3.1 Host galaxy of PS1-13cbe...................... 57 2.3.2 Astrometry.............................. 60 2.3.3 Multi-band Light Curves of PS1-13cbe............... 61 2.3.4 Spectral Features of PS1-13cbe................... 65 2.4 Interpretation of the Features of PS1-13cbe................. 69 2.4.1 Type IIn Supernovae Interpretation of PS1-13cbe.......... 70 2.4.2 PS1-13cbe as a TDE......................... 72 2.4.3 PS1-13cbe as a “Changing Look” AGN............... 74 2.4.3.1 Obscuration of the AGN.................. 75 2.4.3.2 Tidal Disruption Events.................. 77 2.4.3.3 Accretion disk instabilities................ 77 2.4.4 Comparison to other Changing Look AGNs............. 79 2.5 Conclusions.................................. 80 3 PS1-10cdq...................................... 89 3.1 Observations of PS1-10cdq.......................... 89 3.1.1 Optical Photometry.......................... 89 3.1.2 Observations of the Host Galaxy................... 90 3.1.3 X-ray Photometry........................... 90 3.1.4 Optical Spectroscopy......................... 91 3.2 Observational Features of PS1-10cdq..................... 92 3.2.1 Host Galaxy of PS1-10cdq...................... 92 3.2.2 Multi-band Light Curves of PS1-10cdq............... 94 3.2.3 Spectral Features of PS1-10cdq................... 96 3.3 Interpretation of Features of PS1-10cdq................... 107 3.3.1 PS1-10cdq as a Variable AGN.................... 108 3.3.2 PS1-10cdq as SLSN......................... 111 3.3.3 PS1-10cdq as a TDE......................... 113 3.4 Conclusions and Future Remarks....................... 118 4 Galaxy morphology prediction using capsule networks.............. 122 4.1 Introduction.................................. 122 4.2 Galaxy Zoo 2................................. 125 4.3 Related Work................................. 126 4.4 Approach................................... 127 4.4.1 Experimental Setup.......................... 128 4.4.2 Data Preprocessing.......................... 128 4.4.3 Capsule Network........................... 129 4.4.4 Network Architecture......................... 131 4.4.4.1 Capsule Network..................... 131 4.4.4.2 Baseline Network..................... 132 4.4.5 Implementation and Resources.................... 133 9 4.5 Results..................................... 134 4.5.1 Regression.............................. 134 4.5.2 Classification Based on Answers to Question 1 and Reconstruction of Galaxies.............................. 135 4.6 Conclusions.................................. 137 5 Conclusions and Outlook.............................. 142 5.1 Conclusions.................................. 142 5.2 Outlook.................................... 145 References........................................ 147 Appendix: Supplementary Material........................... 178 10 List of Tables Table Page 2.1 Photometry of PS1-13cbe............................. 82 2.1 continued ..................................... 83 2.1 continued ....................................