Nebular Metallicities in Isolated Dwarf Irregular Galaxies
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Nebular Metallicities in Isolated Dwarf Irregular Galaxies David Conway Nicholls A thesis submitted for the degree of Doctor of Philosophy of the Australian National University Research School of Astronomy & Astrophysics July 2014 Notes on the Digital Copy This document makes extensive use of the hyperlinking features of LATEX. References to figures, tables, sections, chapters and the literature can be navigated from within the PDF by clicking on the reference. Internet addresses will be displayed in a browser. Most object names are resolvable in the SIMBAD (http://simbad.u-strasbg.fr/simbad/) astronomical database. The bibliography at the end of this thesis has been hyperlinked to the NASA Astrophysics Data Service (http://www.adsabs.harvard.edu/). Further information on each of the cited works is available through the link to ADS. For Linda. i Disclaimer I hereby declare that the work in this thesis is that of the candidate alone, except where indicated below or in the text of the thesis. The work was undertaken between January 2009 and February 2014 at the Australian National University, Canberra. It has not been submitted in whole or in part for any other degree at this or any other university. Chapter 2 is the paper The Small Isolated Gas-rich Irregular Dwarf (SIGRID) Galaxy Sample: Description and First Results published in the Astronomical Journal, September 2011, volume 142, pp.83 et seq., by David C Nicholls, Michael A Dopita, Helmut Jerjen and Gerhardt R Meurer. The work is entirely that of the candidate. Chapter 3 is the paper Resolving the Electron Temperature Discrepancies in H II Regions and Planetary Nebulae: κ-distributed Electrons published in the Astrophysical Journal, June 2012, volume 752, pp.148 et seq., by David C Nicholls, Michael A Dopita, and Ralph S Sutherland. The work is entirely that of the candidate apart from sections 3.6.1, 3.6.2 and 3.8, which were contributed by Michael Dopita. Chapter 4 is the paper Measuring nebular temperatures: the effect of new collision strengths with equilibrium and κ–distributed electron energies published the Astrophysical Journal Supplement, August 2013, volume 207, pp.21 et seq., by David C Nicholls, Michael A Dopita, Ralph S Sutherland, Lisa J Kewley and Ethan Palay. The work is entirely that of the candidate, with the exception of the data in Tables 3.4 and 3.5, computed by Michael Dopita. Chapter 5 is the paper Nebular metallicities in two isolated Local Void dwarf galaxies, published in the Astrophysical Journal, January 2014, volume 780, pp. 88 et seq., by David C Nicholls, Helmut Jerjen, Michael A Dopita and Hassan Basurah. The work is entirely that of the candidate. Chapter 6 is the paper Metal-poor dwarf galaxies in the SIGRID galaxy sample. I. H ii region observations and chemical abundances, published in the Astrophysical Journal, May 2014, volume 786, pp. 155 et seq., by David C Nicholls, Michael A Dopita, Ralph S Sutherland, Helmut Jerjen, Lisa J Kewley, and Hassan Basurah. The work is entirely that of the candidate. Data for two of the objects presented in this paper were obtained by Michael Dopita. Chapter 7 is the paper Metal-poor dwarf galaxies in the SIGRID galaxy sample. II. The electron temperature–abundance calibration and the parameters that affect it, accepted for publication in the Astrophysical Journal, July 2014, by David C Nicholls, Michael A Dopita, Ralph S Sutherland, Helmut Jerjen, and Lisa J Kewley. The work is entirely that of the candidate. David Conway Nicholls July 2014 iii Acknowledgments In an undertaking such as this, it is impossible to single out all those who have contributed to my achieving this goal. Some, however, have been especially important in guiding me, and I wish to thank them explicitly. First I would like to thank my joint supervisors, Professor Michael Dopita and Dr Helmut Jer- jen. I have been exceptionally fortunate to work with and be guided by two such outstanding scientists. I first met Mike Dopita, when running a government science grants program in the late 1980s. We established a sound working relationship even back then, and Mike was instrumental much later in my applying to study at the RSAA. Mike has been a continuing inspiration and guide in the journey that has led me to complete this thesis. The breadth of Mike’s knowledge in the field of nebular physics is astounding. Without him, both his theoretical knowledge and the brilliant WiFeS spectrograph that he designed and brought into being, this work would have been impossible. My second joint supervisor, Dr Helmut Jerjen, has likewise been profoundly important in guiding me to this goal. Helmut’s understanding of the essence of small galaxies is prodigious. His enthusiasm has been both stimulating and infectious. His guidance in attending to the legion of small but essential details of this work has been critically important. Next I wish to thank Dr. Ralph Sutherland, whose understanding of the intricacies of astrophysical computation is breathtaking. Ralph’s encouragement and guidance in the detailed computation of the atomic data needed for the non-equilibrium calculations helped me with one of the major discoveries in this work. I wish to thank Professor Lisa Kewley for hosting me at the Institute for Astronomy at the University of Hawaii, and for instilling in me the importance of an organised approach to research, a lesson that I have learned well, albeit slowly. I also wish to thank Dr. Mike Childress for his critically important work in developing a new data reduction pipeline for the WiFeS spectroscopic data. I am also indebted to my MSc thesis supervisor, Professor Ted Llewellyn, University of Saskatchewan, who taught me long ago how critically important good writing is to commu- nicating science, at every level. It would be remiss of me also not to mention Professor Don Matthewson, with whom I worked as a research assistant in 1968, and from whom I first grasped what scientific research is about, and who also encouraged me to apply to study at RSAA. Perhaps most important of all, I wish to thank my wife, Linda, for first suggesting in October 2008 that I embark on this wonderful odyssey, and for her patience with my total focus on the work, her continuing encouragement and interest, and for putting up with my endless urges to tell her about the arcane tasks in which I have been involved. Without her support this work would not have been possible. Finally, I wish to thank my colleagues, students and faculty, at the Research School of Astronomy and Astrophysics, Mt Stromlo, for a wonderfully stimulating environment, and for making me realise that I belong in this august company. The stimulating discussions I iv have had with many of the faculty and academic visitors are too numerous to mention. And a special thanks to my talented co-student, Fréd Vogt, whose help, fellowship and sheer joie de la science have contributed to making my studies so immensely enjoyable. There have been so many others who have helped me develop to the point where I was ready for this work. These include my parents, my late first wife, Trish, and my excellent primary school teachers, Iris and Morwood Leyland, St Michael’s School, Putney, where I first developed a fascination with science. And also the European Mole, Talpa europaea, digging in a garden in Walton-on-Thames long ago on a rare, dark, clear English summer’s night, that lured me outside, where I first discovered the glory of the night sky. And the CIA chief of mission in Malaya, in 1959, who showed me the rings of Saturn, the Trapezium and the Orion Nebula, through a 3 inch Edmund Scientific Co. Newtonian telescope. To them, and the legion of others I have not mentioned, I say simply, thank you. v Abstract The motive for this work was to investigate whether small, isolated gas-rich galaxies show evidence of chemical evolution, by studying their nebular metallicities. I have identified a sample of 83 objects chosen for low luminosity and mass, the presence of active star formation, and isolation from other galaxies and galaxy clusters that might generate tidal effects or enrich the intergalactic medium. From these I have measured the spectra of 35 objects, using the WiFeS IFU spectrograph on the ANU 2.3m telescope at Siding Spring. In analysing spectra extracted from the WiFeS data cubes, I found that standard ‘strong line’ methods using emission line ratios to measure atomic abundances, gave either erratic or no results. I found that for those galaxies showing the [O iii] 4363Å auroral line, the metallicities determined using the standard ‘electron temperature’ method were inconsistent with previous published work. This led me to investigate the conventional assumption that electrons in H ii regions are in thermal equilibrium. I show that the non-equilibrium ‘κ’ electron energy distribution, found almost universally in solar system plasmas, can explain the long recognised ‘abundance discrepancy’ between recombination line and collisional line abundance calculations in nebular metallicity measurements. This has added an important new dimension to the analysis of nebular spectra. Using the extensively revised Mappings photoionisation modelling code and new atomic data to analyse the spectra of two exceptionally isolated dwarf galaxies, I find that they exhibit metallicities similar to galaxies in more crowded environments, and appear to have evolved quite normally, through periodic star formation and subsequent enrichment of their interstellar media. I present a new approach for calculating total oxygen abundance using electron temperat- ures that appears to give more consistent results than earlier methods. I apply this to my measured spectra, together with the revised Mappings photoionisation modelling code, to explore the physical parameters affecting the measurement of nebular metallicities.