Diffuse Gamma-Ray Emission from Nearby Molecular Clouds As a Probe of Cosmic Ray Density Variations
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City University of New York (CUNY) CUNY Academic Works All Dissertations, Theses, and Capstone Projects Dissertations, Theses, and Capstone Projects 2-2017 Diffuse Gamma-Ray Emission from Nearby Molecular Clouds as a Probe of Cosmic Ray Density Variations Ryan Abrahams Graduate Center, City University of New York How does access to this work benefit ou?y Let us know! More information about this work at: https://academicworks.cuny.edu/gc_etds/1881 Discover additional works at: https://academicworks.cuny.edu This work is made publicly available by the City University of New York (CUNY). Contact: [email protected] Diffuse Gamma-ray Emission from Nearby Molecular Clouds as a Probe of Cosmic Ray Density Variations by Ryan Douglas Abrahams A dissertation submitted to the Graduate Faculty in Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy, The City University of New York 2017 ii c 2017 Ryan Douglas Abrahams All Rights Reserved iii This manuscript has been read and accepted by the Graduate Faculty in Physics in satisfaction of the dissertation requirement for the degree of Doctor of Philosophy. Professor Timothy Paglione Date Chair of Examining Committee Professor Igor Kuskovsky Date Executive Officer Professor Saavik Ford Professor Barry McKernan Professor Reshmi Mukherjee Professor Matthew O'Dowd Supervisory Committee The City University of New York iv Abstract Diffuse Gamma-ray Emission from Nearby Molecular Clouds as a Probe of Cosmic Ray Density Variations by Ryan Douglas Abrahams Adviser: Timothy Paglione We analyze diffuse gamma-ray emission from nearby molecular clouds in order to probe properties of the interstellar medium and the distribution of high energy cosmic rays. Diffuse gamma-ray emission from interstellar clouds results largely from cosmic ray proton collisions with ambient gas, regardless of the gas state, temperature, or dust properties of the cloud. The interstellar medium is predominantly transparent to both cosmic rays and gamma-rays, so GeV emission is a unique probe of the total gas column density. The gamma-ray flux per column density, or emissivity, of a cloud with known column density is then a measure of the impinging cosmic ray population and may be used to map the kiloparsec-scale cosmic ray distribution in the Galaxy. In this thesis, we test a number of commonly used column density tracers to evaluate their effectiveness in modeling the GeV emission from the relatively quiescent, nearby ρ Ophiuchi molecular cloud. We confirm that a single tracer of the interstellar medium does not reproduce the total gas golumn densities probed by diffuse gamma-ray emission. Instead, both H I and an appropriate H2 tracer, at minimum, are required model the gas column density. We find that the optical depth at 353 GHz (τ353) from Planck reproduces the gamma-ray data best overall based on the test statistic across the entire region of interest, but near infrared stellar extinction also performs very well, with smaller spatial residuals in the densest parts of the cloud. v Then, we analyze the gamma-ray emission, taken with the Fermi Large Area Telescope between 250 MeV and 10 GeV, from 93 sites at high Galactic latitude with CO emission. Observations of gamma-rays allow us to probe the density and spectrum of cosmic rays in the solar neighborhood. The clouds studied lie within ∼270 pc from the Sun and are selected from the Planck all-sky CO map. For these regions, we trace the interstellar medium with three components: H I , CO, and dark gas, which is CO-faint-H2 . We detect gamma-ray emission from the H2 phases in the majority of regions and many regions show significant, extended gamma-ray emission from the molecular gas. The gamma-ray emission is domi- nated by the CO-emitting gas in some clouds, but by the CO-faint gas in others. Neither the gas emissivities nor the CO-to-H2 conversion factors show any evidence of systematic variation. However, the CO-to-H2 factor shows significant intracloud variation. The large uncertainties and variance in emissivity does not rule out any physically relevant models of cosmic ray source distribution or propagation, but the intracloud CO-to-H2 variation is similar to that predicted by molecular cloud models. Finally, we comment on additional observations and analysis methods which may be applied to the study of interstellar medium and cosmic ray physics. Acknowledgments I would like to thank my advisor Timothy Paglione, who showed what felt like extreme patience and expert guidance, especially when I went on some theoretical tangents. He has encouraged me to actually finish things and influenced the way I approach problems. I would like to thank the members of my thesis committee for insights and the time spent and the astronomy faculty of both CUNY and those from Colubmia with whom I have spoken. And thank you to everyone at the American Museum of Natural History for creating a welcoming and challenging environment. Thank you, Filis Coba, for inspiring me to continue growing, and for showing me better ways of looking at teaching, explaining myself, and thinking. Thanks to my family for providing support and allowing me a welcome reprieve from the craziness of studying and New York City every once in a while. Thank you, too, for actually reading some of the stuff I wrote on my blog. It included a lot of nearly stream-of-consciousness techno-babble. Sifat Chowdhury and John Raderman, it's been fun, thanks for being there. The past 6.5 years has been an interesting time. I've been exposed to a host of different subjects, and I find many of them fascinating. Thanks to Alex Teachey for the excellent work and conversations. I would like to thank Dr. Greenbaum and Dr. Kuskovsky for their leadership of the CUNY physics Ph.D. program and Daniel Moy for making sure the program is running. I want to thank all of the students with whom I have talked. I'd like to mention especially Steve Vayl, Matt Moocarme, Tom Proctor, Zhibai Zhang, Arthur Parzygnat, and vi vii the rest of the high energy journal club group for joining me in my half-baked seminars and study groups in quantum gravity, the foundations of physics, and machine learning. They are better thinkers than I, and I have absorbed a lot from talking to all of them. And Matt Grasser who helped show me what is actually possible with a physics education and some genuine interest. We've had a lot of great conversations on physics, the future, and the real world. I have had a few inspiring conversations with Doug Finkbeiner and his students Tansu Daylan, Stephen Portillo, all of whom I met at the 2013 Fermi Summer School. And I have talked with Ron Allen about careers, ISM physics, and dark gas. I am grateful to the University of Indiana, Bloomington and Alan Kosteleck´yfor holding a tuition-free summer school on Lorentz violating extensions to the Standard Model. I never did get around to testing those coefficients with GRBs... I am grateful to the Fermi science team for organizing the summer program and the symposium. And to Carlo Rovelli for having the patience to talk to me about primordial black holes and observational signatures of quantum gravity after I cornered him after he gave a public talk. Thank you to Cole Miller for helping shape my interest in observational high energy astronomy and dealing with me while I was a tiny undergraduate. Lloyd Bumm, who hosted me in 2009 for my REU at the University of Oklahoma. It was a fun two months, but that was enough Oklahoma for me! I did take a lot of good skills from there, though. And Todd Rosenfeld who taught my 11th grade astronomy class. That class kept my interest in astronomy going strong. I am glad I am finishing this lengthy adventure. I've seen, and even helped recruit, a lot of students in the CUNY Graduate Center. I have taught more than I thought I ever would, and thought long and hard about a huge variety of subjects: high energy astrophysics, even higher energy physics, Galactic structure, ISM physics, mathematical methods (e.g., my point-free geometry nonsense), quantum gravity (loops and strings and fuzzy spheres viii and graphs), machine learning ... This chapter of my life, described in the following seven chapters, is coming to a close. I am grateful for those who have supported me and with whom I have talked, and I am looking forward to what comes next. Contents Contents ix List of Tables xii List of Figures xiii 1 Introduction 1 1.1 CRs and Star Formation . 3 1.2 CRs in the Milky Way Galaxy . 4 1.3 Thesis Outline . 5 2 The Physics of CRs and Gamma-rays 6 2.1 Cosmic Ray Sources . 7 2.2 Cosmic Ray Diffusion . 8 2.3 Gamma-ray Emission Mechanisms . 11 2.4 Molecular Clouds . 12 2.5 Fermi LAT Instrument . 15 2.6 Gamma-ray Observations of ISM . 16 2.6.1 Gas Composition Results . 17 2.6.2 Cosmic Ray Results . 19 ix CONTENTS x 3 Analysis with Fermi 21 3.1 Gamma-ray Data . 21 3.2 Gamma-ray Analysis . 22 3.3 Results of the Likelihood Fit . 27 4 Gamma-ray Model Components 29 4.1 Point Sources . 29 4.2 Fermi Galactic Diffuse Emission Model . 30 4.3 Diffuse Spatial Maps . 34 4.4 Emissivities and Gas Properties . 40 4.4.1 XCO Factor . 40 4.4.2 Xdust Factor . 40 5 Calibrating Total Column Density 42 5.1 Introduction . 42 5.2 Gamma-ray Data and Modeling .