Using Emission Lines to Characterize Galaxy Properties: From the Nearby to the Distant Universe

Open Access
Bridge, Joanna S
Graduate Program:
Astronomy and Astrophysics
Doctor of Philosophy
Document Type:
Date of Defense:
August 18, 2017
Committee Members:
  • Caryl Ann Gronwall, Dissertation Advisor/Co-Advisor
  • Caryl Ann Gronwall, Committee Chair/Co-Chair
  • Robin Bruce Ciardullo, Committee Member
  • Michael Eracleous, Committee Member
  • Christopher Howard House, Outside Member
  • Donald P Schneider, Committee Member
  • astronomy
  • astrophysics
  • galaxies
  • emission lines
  • star formation
Ever since the hydrogen emission line of Lyman-alpha (Lyα) has been postulated to hold one of the keys to studying high-redshift galaxies, the question of what emission lines can tell us about the properties and evolution of galaxies has driven much of our research. This work is a survey of emission lines across cosmic history. Just as galaxies evolve with redshift, so does how we can use emission line information from these galaxies. I begin with a study of resolved Lyα emission in nearby galaxies, and how we can use pixel-by-pixel photometry to study emission on very small galactic scales and to characterize Lyα scattering. Using the Lyman-Alpha Reference Sample, a set of 14 starbursting galaxies with redshifts of 0.02 < z < 0.2 observed with the Hubble Space Telescope (HST ) Solar Blind Channel on the Advanced Camera for Surveys, I have characterized how the Lyα photons scatter. Since Lyα is a resonant line, the photons are absorbed and re-emitted by the neutral hydrogen in galaxies until they finally escape or are absorbed by dust. Understanding how this random walk process affects where and how much Lyα we observe is important for us to be able to use Lyα as a signpost of galaxy evolution at high redshifts. By comparison, the Hα transition, which is optically thin and therefore where we observe Hα photons are likely where they were emitted originally, is much easier to trace. I have developed a method to compare the Hα and Lyα distributions in galaxies to determine a characteristic scattering length for Lyα photons. This is an important step in understanding exactly how much a Lyα photon scatters before escaping, and is important for understand the overall escape fraction of Lyα. Shifting to higher redshifts of z ∼ 0.5, emission lines can no longer be used to study the emission from galaxies at such high resolution. Instead, I examine how we can study global galaxy properties using the emission lines, and how we can relate that to galaxy size and morphology. For this study, I used a sample of 284 [O II] emitting galaxies from the Hobby Eberly Telescope Dark Energy Survey (HETDEX) Pilot Survey. Using the forbidden transition of [O II] as a star-formation rate indicator, we found that these galaxies corroborate the main sequence of star forming galaxies. I confirm, however, that the [O II] line is very sensitive to metallicity and abundance. I find that the star formation in these galaxies is generally not due to galaxy mergers, but more likely the result in situ cold gas accretion. Galaxy emission activity becomes more complicated as we approach cosmic noon (z ∼ 2), when star formation was at its peak. To address this issue, I look at how we can use both Balmer and oxygen emission lines to help disentangle star formation activity from emission caused by active galactic nuclei. I do this by simulating z ∼ 2 galaxies with Hβ at 4861 Angstroms and [O III]λ5007 emission and performing mock HST grism observations with the infrared G141 grism on the Wide Field Camera 3 (WFC3). The benefit of observing these galaxies using a grism is that we are able to gather spatial information about where the emission is coming from, whether it is nuclear or from more extended regions. By developing a method for separating these emission regions in our observations, we provide a way to find obscure or low-mass active galactic nuclei (AGN) that are undetectable by other methods. Finally, I explore our prospects for using emission lines to probe the first galaxies at z > 6 and how we can leverage new technology to catch them in the act of formation. I examine the completeness of the WFC3/G102 grism program CANDELS Lyman-alpha Emission at Reionization (CLEAR) survey, simulating Lyα emission at these high redshifts. This is a necessary step in being able to leverage the ∼ 90 Lyα emitters that will be found to tell us not only about galaxy formation at high redshift but also about the luminosity function of Lyα emitters during this cosmic era.