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UC Santa Cruz UC Santa Cruz Electronic Theses and Dissertations Title Searching for the Lowest Metallicity Galaxies in our Local Universe Permalink https://escholarship.org/uc/item/1333k6vz Author Hsyu, Tiffany Publication Date 2020 Peer reviewed|Thesis/dissertation eScholarship.org Powered by the California Digital Library University of California UNIVERSITY OF CALIFORNIA SANTA CRUZ SEARCHING FOR THE LOWEST METALLICITY GALAXIES IN OUR LOCAL UNIVERSE A dissertation submitted in partial satisfaction of the requirements for the degree of DOCTOR OF PHILOSOPHY in ASTRONOMY AND ASTROPHYSICS by Tiffany Hsyu June 2020 The Dissertation of Tiffany Hsyu is approved: Michael Bolte, Chair Ryan J. Cooke J. Xavier Prochaska Quentin Williams Acting Vice Provost and Dean of Graduate Studies Copyright c by Tiffany Hsyu 2020 Table of Contents List of Figures vi List of Tables xii Abstract xiv Acknowledgments xvi Dedication xx 1 Introduction1 1.1 Big Bang Nucleosynthesis..........................3 1.1.1 BBN predictions and the Standard Model.............6 1.2 The observational primordial abundances.................8 1.2.1 Helium-3, 3He.............................9 1.2.2 Lithium-7, 7Li............................ 11 1.2.3 Deuterium, D............................. 13 1.2.4 Helium-4, 4He............................. 14 1.2.5 Confronting the Standard Model.................. 17 1.3 Outline of this work............................. 19 2 Searching for new, metal-poor dwarf galaxies in the local Universe 20 2.1 Introduction.................................. 20 2.2 Candidate low-metallicity galaxy selection from SDSS.......... 26 2.2.1 Photometric color-color selection.................. 26 2.2.2 Morphological selection....................... 27 2.3 Spectroscopic observations.......................... 31 2.3.1 Lick Observatory........................... 31 2.3.2 Keck Observatory.......................... 34 2.3.3 Data reduction............................ 35 2.4 Analysis.................................... 37 2.4.1 Emission line flux measurements.................. 37 2.4.2 Metallicity measurements...................... 44 iii Lick data { metallicity via the R and S calibration method... 44 Keck data { metallicity via the direct method........... 47 2.4.3 R and S calibration versus direct metallicity measurements... 54 2.5 Discussion................................... 57 2.5.1 Distance, Hα luminosity, and star formation rate......... 57 2.5.2 Stellar mass.............................. 59 2.5.3 The luminosity-metallicity relation for metal-poor galaxies.... 60 2.5.4 The mass-metallicity relation for metal-poor galaxies....... 64 2.5.5 The search for metal-poor galaxies in other photometric surveys 67 2.6 Conclusion.................................. 70 3 The Little Cub 72 3.1 Introduction.................................. 72 3.2 Spectroscopic observations.......................... 74 3.2.1 Lick Observatory........................... 74 3.2.2 Keck Observatory.......................... 75 3.2.3 Data reduction............................ 75 3.3 Analysis.................................... 77 3.3.1 Emission line flux measurements.................. 77 3.3.2 Metallicity measurements...................... 82 3.4 Discussion................................... 85 3.4.1 Distance and physical properties.................. 85 3.4.2 Environment............................. 87 3.5 Conclusion.................................. 89 4 The primordial helium abundance 91 4.1 Introduction.................................. 91 4.2 Observations, data reduction, and analysis................. 98 4.2.1 Keck Observatory.......................... 98 Optical spectroscopy......................... 100 Near-infrared spectroscopy..................... 101 4.2.2 Data reduction............................ 101 4.2.3 SDSS sample............................. 103 4.2.4 Emission line measurements..................... 104 4.2.5 HeBCD sample............................ 106 4.3 Model overview................................ 109 4.3.1 Emissivities.............................. 113 Hydrogen emissivity......................... 114 Helium emissivity.......................... 115 C 4.3.2 Collisional to recombination ratio, R (λ).............. 115 C Hydrogen R (λ)............................ 115 C Helium R (λ)............................. 118 4.3.3 Underlying stellar absorption.................... 119 Hydrogen absorption......................... 120 Helium absorption.......................... 121 iv 4.3.4 Reddening correction......................... 122 4.3.5 Optical depth function........................ 122 4.3.6 Markov Chain Monte Carlo (MCMC) details........... 123 4.4 The primordial helium abundance..................... 128 4.4.1 Sample qualification......................... 128 Sample 1............................... 128 Sample 2............................... 128 4.4.2 Abundance measurements...................... 130 Oxygen................................ 132 Helium................................ 133 4.4.3 Extrapolation to yP ......................... 135 4.4.4 Comparison to existing measurements of YP ............ 143 4.5 Discussion................................... 146 2 4.5.1 Implications for the Standard Model { BBN bounds on Ωbh and Neff .................................. 146 4.5.2 Future improvements......................... 150 Qualification rates.......................... 150 Towards a sub-percent measurement of yP ............. 153 4.6 Conclusion.................................. 156 5 Summary and future directions 160 5.1 Summary................................... 160 5.2 Future directions............................... 162 5.2.1 Where are all the near-pristine galaxies?.............. 162 5.2.2 Where do HII region models fail?.................. 165 Bibliography 169 A Appendix 180 A.1 SDSS CasJobs query for candidate metal-poor galaxies via photometry. 180 A.2 SDSS CasJobs query for spectra of candidate systems to be included in primordial helium determination...................... 181 A.3 Mock Data and MCMC Recovery...................... 182 v List of Figures 1.1 The dependence of the primordial number abundance ratios, relative to hydrogen, on the baryon-to-photon ratio, η10 (also shown above as the 2 baryon density Ωbh ). The upper panel shows the predicted primordial number abundance ratios for 4He the middle panel shows the values for 3He and D=H, and the lower panel shows the values for 7Li. The red, green, and blue curves represent the predictions assuming a number of neutrino species Nν = 2, 3 (the Standard Model value), and 4, respectively. The widths of the curves indicate the 1σ errors on the predictions. The grey vertical bar indicates the 1σ constraint on the baryon density as derived from observations of the CMB by the Planck satellite, as quoted in Section 1.1.1. This figure is modelled after e.g., Figure 7 of Cyburt et al. 2016 and Figure 26 Pitrou et al. 2018, using data provided by Ken Nollett via email. For details of this code, see Nollett & Burles 2000 and Cooke et al. 2016................................7 2 1.2 The precision to which the baryon density, Ωbh , and effective number of neutrino species, Neff , can be measured, given the precision of the determination of the primordial abundances. The plateauing of the curves is a reflection of uncertainties in the reaction rates that are used in BBN calculations. Given the current best measurements of the primordial D=H and 4He abundances, approximately 1 per cent or log ∆p=p = 2, D=H 2 − provides the strongest constraints on the both Ωbh and Neff . However, the payoff for further increasing the accuracy of D=H will soon plateau, whereas pushing 4He to the sub-per cent level will provide an increasingly powerful constraint on Neff .......................... 10 vi 2.1 Our SDSS g r color selection criteria versus u g, r i, and i z color − − − − cuts in the upper, middle, and lower panels, respectively. The purple points represent the location in color-color space of candidate metal-poor galaxies selected for observing. The blue points show the location of the known, extremely metal-poor systems such as Leo P and I Zwicky 18 (both the northwest and southeast components), in the same color-color space. Error bars on the colors are shown. We note that Leo P and I Zwicky 18 were known systems prior to this survey and helped define our color-color search criteria, whereas AGC198691 and the Little Cub were identified as a result of the query. The lowest-metallicity systems appear to cluster around u g 0:27 and i z 0:06, with the exception of − ∼ − ∼ − Leo P in the latter............................... 29 2.2 SDSS imaging of Leo P and I Zwicky 18, two of the most metal-poor H ii regions currently known, are shown in the left and middle panels of the upper row. The remaining panels show SDSS imaging of seven H ii regions selected for observing via our photometric method and predicted to be of low-metallicity. Spectra corresponding to the new systems are shown in Fig 2.3. The images are shown on the same angular scale of 1500 on a side, with north up and east to the left................ 30 2.3 Discovery spectra (shown in black) obtained using the Shane 3-m tele- scope at Lick Observatory of seven H ii regions in our sample that are predicted to have the lowest metallicities via the R and S calibration methods. The error spectra are shown in red. Emission lines of interest for the R and S calibration methods are labeled in the first panel. The gap between 5400{5900 A˚ in all panels is due to the d55 dichroic used ∼ during our observations on

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