Copyright by John Caleb Barentine 2013 the Dissertation Committee for John Caleb Barentine Certifies That This Is the Approved Version of the Following Dissertation
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Copyright by John Caleb Barentine 2013 The Dissertation Committee for John Caleb Barentine certifies that this is the approved version of the following dissertation: The Role Of Gas In Galaxy Evolution: Infall, Star Formation, And Internal Structure Committee: John Kormendy, Supervisor Shardha Jogee Edward L. Robinson Gregory Shields Bart P. Wakker The Role Of Gas In Galaxy Evolution: Infall, Star Formation, And Internal Structure by John Caleb Barentine, B.S., M.S., M.A. DISSERTATION Presented to the Faculty of the Graduate School of The University of Texas at Austin in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY THE UNIVERSITY OF TEXAS AT AUSTIN May 2013 This work is dedicated to the memory of my father, John Bruce Barentine (11 August 1952 − 17 January 2010) Acknowledgments I give thanks to many people whose contributions, support, and com- miseration have made this work possible and helped sustain my efforts long after I stopped believing in myself. In 2001, the journal Science published an editorial1 titled \An Algo- rithm For Discovery" in which two neuroscientists from UMass tried to some- how parameterize the process of science and distill it into \a kind of flow chart that could be carried on a laminated card." A photocopy of this article hung on my office door for a long time. The authors' conclusions, distilled into a series of bullet points, reads: 1. Slow down to explore. 2. Read, but not too much 3. Pursue quality for its own sake. 4. Look at the raw data. 5. Cultivate smart friends. I learned all five of these elements and their importance from different people along the way. 1D. Paydarfar and W.J. Schwartz, 2001, Science, 292, 5514, 13 v To my supervisors in grad school, Drs. John Kormendy, John Lacy, and Bart Wakker. The contents herein bear your indelible marks. I also thank Drs. Shardha Jogee, Rob Robinson, and Greg Shields for their service on my supervising committee. (2, 3, 4) To my parents, John and Delsia Barentine, for never once questioning my ridiculous decision to go into science as a career choice. They often didn't understand what I did for a living, but they were no less proud as a result. One of the greatest disappointments in life is that my father did not live to see this work completed. I have done my best to honor his memory as I finished up this work in his absence. (1, 2, 3) To my friends, too numerous to name individually, for their counsel and sympathy (and for not talking me out of grad school). I would be remiss if I didn't thank the staff of Texas Coffee Traders at Robert Lee Moore Hall, who literally fueled this work with caffeine for several years (in particular, Candice Dublin, Patrick Edwards, and Joseph Duke). (5) To my class, Amanda Bayless, Guillermo Blanc, Sean Couch, Bi- Qing For, Candace Gray, Amanda Heiderman, Randi Ludwig, Jeremy Murphy, Sehyun Hwang, Hyo-Jeong Kim, Masatoshi Shoji, and Tim Weinzirl, I owe a debt for years of reassurance, camaraderie, and commiseration as needed. No one could ask for a better cohort with which to endure this experience. We turned mostly okay, didn't we? (3, 4, 5) To former mentors in science, Drs. Roger Culver, Stuart Field, vi Ian Gatley, Karen Harvey, Charlie Lindsey, Stuart Jeffries, K. Mike Merrill, and Caty Pilachowski, from whom I learned the nuts and bolts fundamentals of how science is done, for teaching me the craft of science. (1, 2, 3, and especially 4) To my principal counselor throughout nearly the entire grad school experience, Dr. James Gay, who proved to me that attitude really is everything (3, 5). In 1936, in Physics and Reality, Einstein wrote, \The whole of science is nothing more than a refinement of everyday thinking.' Practicing this art does not require elaborate instrumentation, generous funding, or prolonged sabbaticals. What it does require is a commitment to exercising one's creative spirit { for curiosity's sake." However, Einstein also famously quipped, \Sci- ence is a wonderful thing if one does not have to earn one's living at it." John Barentine Austin April 2013 vii The Role Of Gas In Galaxy Evolution: Infall, Star Formation, And Internal Structure Publication No. John Caleb Barentine, Ph.D. The University of Texas at Austin, 2013 Supervisor: John Kormendy The story of a typical spiral galaxy like the Milky Way is a tale of the transformation of metal-poor hydrogen gas to heavier elements through nuclear burning in stars. This gas is thought to arrive in early times during the assembly phase of a galaxy and at late times through a combination of hot and cold “flows" representing external evolutionary processes that continue to the present. Through a somewhat still unclear mechanism, the atomic hydrogen is converted to molecules that collect into clouds, cool, condense, and form stars. At the end of these stars' lives, much of their constituent gas is returned to the galaxy to participate in subsequent generations of star formation. In earlier times in the history of the universe, frequent and large galaxy mergers brought additional gas to further fuel this process. However, major merger activity began an ongoing decline several Gyr ago and star viii formation is now diminishing; the universe is in transitioning to an era in which the structural evolution of disk galaxies is dominated by slow, internal (\secular") processes. In this evolutionary regime, stars and the gas from which they are formed participate in resonant gravitational interactions within disks to build ephemeral structures such as bars, rings, and small scale-height central bulges. This regime is expected to last far into the future in a galaxy like the Milky Way, punctuated by the periodic accretion of dwarf satellite galaxies but lacking in the \major" mergers that kinematically scramble disks into ellipticals. This thesis examines details of the story of gas from infall to structure-building in three major parts. The High- and Intermediate-Velocity Clouds (HVCs/IVCs) are clouds of H i gas at velocities incompatible with simple models of differential Galactic rotation. Proposed ideas explaining their observed properties and origins in- clude (1) the infall of low-metallicity material from the Halo, possibly as cold flows along filaments of a putative \Cosmic Web"; (2) gas removed from dwarf satellite galaxies orbiting the Milky Way via some combination of ram pressure stripping and tidal disruption; and (3) the supply and return feeds of a \Galac- tic Fountain" cycling gas between the Disk and Halo. Numerical values of their observed properties depend strongly on the Clouds' distances. In Chapter 2, we summarize results of an ongoing effort to obtain meaningful distances to a selection of HVCs and IVCs using the absorption-line bracketing method. We find the Clouds are not at cosmological distances, and with the exception of the Magellanic Stream, they are generally situated within a few kiloparsecs ix of the Disk. The strongest discriminator of the above origin scenarios are the heavy element abundances of the Clouds, but to date few reliable Cloud metal- licities have been published. We used archival UV spectroscopy, supplemented by new observations with the Cosmic Origins Spectrograph aboard the Hubble Space Telescopeand H i 21 cm emission spectroscopy from a variety of sources to compute elemental abundances relative to hydrogen for 39 HVC/IVC com- ponents along 15 lines of sight. Many of these are previously unpublished. We find support for all three origin scenarios enumerated above while more than doubling the number of robust measurements of HVCs/IVCs in existence. The results of this work are detailed in Chapter 3. In Chapter 4 we present the results of a spectroscopic study of the high-mass protostellar object NGC 7538 IRS 9 made with the Texas Echelon Cross Echelle Spectrograph (TEXES), a sensitive, high spectral resolution, mid-infrared grating spectrometer and compare our observations to published data on the nearby object NGC 7538 IRS 1. Forty-six individual lines in vibra- tional modes of the molecules C2H2, CH4, HCN, NH3 and CO were detected, 13 12 18 including two isotopologues ( CO, C O) and one combination mode (ν4 +ν5 C2H2). Fitting synthetic spectra to the data yielded the Doppler shift, exci- tation temperature, Doppler b parameter, column density and covering factor for each molecule observed; we also computed column density upper limits for lines and species not detected, such as HNCO and OCS. We find differences among spectra of the two objects likely attributable to their differing radia- tion and thermal environments. Temperatures and column densities for the x two objects are generally consistent, while the larger line widths toward IRS 9 result in less saturated lines than those toward IRS 1. Finally, we compute an upper limit on the size of the continuum-emitting region (∼2000 AU) and use this constraint and our spectroscopy results to construct a schematic model of IRS 9. In Chapters 5 and 6, we describe studies of the bright, nearby, edge- on spiral galaxies NGC 4565 and NGC 5746, both previously classified as type Sb spirals with measured bulge-to-total luminosity ratios B=T ' 0:4. These ratios indicate merger-built, \classical" bulges but in reality represent the photometric signatures of bars seen end-on. We performed 1-D photomet- ric decompositions of archival Hubble Space Telescope, Spitzer Space Telescope, and Sloan Digital Sky Survey images spanning a range of wavelengths from the optical to near-infrared that penetrate the thick midplane dust in each galaxy. In both, we find high surface brightness, central stellar components that are clearly distinct from the boxy bar and from the disk; we interpret these structures as small scale height \pseudobulges" built from disk material via internal, resonant gravitational interactions among disk material − not classical bulges.