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E+A Galaxy and Spiral E+A Galaxy Candidates in the Hercules Supercluster

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E+A Galaxy and Spiral E+A Galaxy Candidates in the Hercules Supercluster E+A galaxy and spiral E+A galaxy candidates in the Hercules supercluster by Rosemary Williams A THESIS submitted to Oregon State University Honors College in partial fulfillment of the requirements for the degree of Honors Baccalaureate of Science in Geophysics (Honors Scholar) Presented March 5, 2021 Commencement June 2021 2 3 AN ABSTRACT OF THE THESIS OF Rosemary Williams for the degree of Honors Baccalaureate of Science in Geophysics presented on March 5, 2021. Title: E+A galaxy and spiral E+A galaxy candidates in the Hercules supercluster. Abstract approved:____________________________________________________ Charles Liu E+A galaxies represent an important niche in galaxy evolution as a subset of post- starburst galaxies. Set apart from other post-starburst galaxies by their rapid quenching of star formation, E+As are thought to have started their formation outside a galaxy cluster's center and have fallen inward due to gravity; the star forming gas is ram-pressure stripped from the galaxy, quenching star formation and leaving behind a post-starburst galaxy. This galaxy has a high population of older, redder stars from the pre-starburst galaxy, and also contains new, blue, A-type stars that formed during the merger. This paper investigates E+As within the Hercules supercluster, and pays particular attention to Khutulun (SDSS 2MASX J16015198+1547326), a perfect example of a barred spiral (SBb type) E+A galaxy, and the inspiration for investigating spiral E+As. Spiral E+As represent an even smaller subgroup of galaxies as they have retained their arms through starburst, and thus were most likely formed without a major galaxy merger. This research presents theories on how spiral E+As formed and supports current theories that E+As tend to be in low/medium density regions within clusters and provides evidence that perhaps the gas density region is correlated to how the galaxy underwent starburst. Key Words: galaxies, evolution, post-starburst, E+A, spiral E+A, Hercules Corresponding e-mail address: [email protected] 4 ©Copyright by Rosemary Williams March 5, 2021 5 E+A galaxy and spiral E+A galaxy candidates in the Hercules supercluster by Rosemary Williams A THESIS submitted to Oregon State University Honors College in partial fulfillment of the requirements for the degree of Honors Baccalaureate of Science in Geophysics (Honors Scholar) Presented March 5, 2021 Commencement June 2021 6 Honors Baccalaureate of Science in Geophysics project of E+A galaxy and spiral E+A galaxy candidates in the Hercules supercluster presented on March 5, 2021. APPROVED: _____________________________________________________________________ Charles Liu, Mentor, representing the American Museum of Natural History _____________________________________________________________________ Randall Milstein, Committee Member, representing Physics _____________________________________________________________________ Xavier Siemens, Committee Member, representing Physics _____________________________________________________________________ Toni Doolen, Dean, Oregon State University Honors College I understand that my project will become part of the permanent collection of Oregon State University, Honors College. My signature below authorizes release of my project to any reader upon request. _____________________________________________________________________ Rosemary Williams, Author 7 I would like to thank Dr. Charles Liu for mentoring me through the research process and being one of the most understanding and caring mentors I have ever met. You supported me through the largest research project I have ever tackled, and your constant enthusiasm and encouragement was delightful. Thank you to Dr. Randall Milstein for being a mentor to me since my freshman year and pushing me to apply to so many different opportunities that I never thought I could actually get. Thank you for all your letters of recommendation and for being a member on my thesis committee and providing insightful, critical feedback. Thank you to Dr. Siemens for graciously agreeing to be on my committee without having ever even met me before, I appreciate all of the time you dedicated to this thesis especially given all of your amazing research projects you are currently juggling. I would also like to thank the Y/Dim Collaboration for being such an amazing and supportive research group. I have had such an amazing time getting to know you all and I have learned so much from you. I would also like to acknowledge the NSF REU program at the American Museum of Natural History for funding this research, and to everyone at the museum who worked tirelessly so the program could go on virtually during a pandemic. This work was supported by the Alfred P. Sloan Foundation via the SDSS-IV Faculty and Student Team (FaST) initiative, ARC Agreement SSP483, and by NSF grants AST-1852355, 1852360, 1460939, and 1460860 to the American Museum of Natural History and CUNY College of Staten Island. 8 Table of Contents Table of Contents ....................................................................................................... 8 Chapter 1: Introduction ......................................................................................... 9 Section 1.1 Background and history ........................................................................... 9 Section 1.2 The science behind E+A galaxies ............................................................ 12 Chapter 2: Data acquisition ................................................................................. 16 Chapter 3: Results and Analysis ........................................................................... 22 Section 3.1 E+A Results and Analysis ........................................................................ 23 Section 3.2 Spiral E+A Results and Analysis .............................................................. 26 Section 3.3 Cold and hot ICM regions within Hercules ............................................. 28 Chapter 4: Discussion .......................................................................................... 30 Section 4.1 How do spiral E+As form? ...................................................................... 30 Section 4.2 ICM hot and cold gas analysis ................................................................ 33 Chapter 5: Considerations ................................................................................... 35 Chapter 6: Conclusion .......................................................................................... 37 Chapter 7: Works Cited ....................................................................................... 38 Chapter 8: Appendix ............................................................................................ 41 Section 8.1 Equivalent width code ............................................................................ 41 Section 8.2 NN code .................................................................................................. 42 Section 8.3 3D visualizations of elliptical and spiral E+A galaxies ............................ 43 Section 8.4 Example of SDSS classified E+A that is not fully E+A .............................. 44 9 Chapter 1: Introduction Section 1.1 Background and history In 1983, Dressler and Gunn identified what would become the first known 'E+A' galaxies while studying the shape of galaxy spectra in the 3C 295 cluster [1]. The spectra that caught their attention lacked star formation and seemed to contain two contradictory star populations: old, red, K-type stars and a considerable population of new, blue, A- type stars. This unlikely combination of stellar populations forced the galaxies’ apparent color into a “green valley” (see Figure 1): bluer than a typical elliptical galaxy and redder than a typical spiral galaxy. They were called “E+A” (or “K+A”) because their galaxy spectra could be modelled using typical “E”-lliptical galaxies (which contain high populations of “K”-type stars) and “A”-type stars. Figure 1: Color-mass diagram illustrating the location of the green valley from Kevin Schawinski et al [2]. 10 One explanation for how these galaxies formed is a past event triggered a starburst (a period of star formation ~10 times higher than in the average star-forming galaxy) within the galaxy that led to the formation of young, blue stars among the red stars of the host galaxy. Overtime star formation ended, most likely due to star-forming gas being blown out of the galaxy as it moved through its galaxy cluster in a process called ram-pressure stripping (RPS), leaving the galaxy quiescent and E+A. But what triggered these starbursts? In 1988, Lavery and Henry presented spectrographic and photographic evidence of galaxy-galaxy interactions at around redshift -.2 that had triggered starbursts within galaxies [3]. In 1991, Oegerle, Hill, and Hoessel identified the irregular galaxy “G515” (pictured in Figure 2), the first low-redshift E+A galaxy discovered (z = 0.0875), and an important discovery because its high luminosity (rare for elliptical galaxies) and distinct morphology suggested evidence for a recent merger [4]. Figure 2: G515 (left) was one of the first E+A galaxies studied in detail and Khutulun (right) is the spiral E+A galaxy that inspired this research. Note the faint trail of material swinging beneath G515 and the irregular shape of the center alluding to a past merger or harassment event. Images taken from SDSS. 11 While mergers had been a possibility for how E+A galaxies formed,
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