Multi-Phase Modeling of Intervening Quasar Absorption Line Systems at z < 1

Open Access
Norris, Jackson
Graduate Program:
Astronomy and Astrophysics
Doctor of Philosophy
Document Type:
Date of Defense:
June 07, 2019
Committee Members:
  • Jane Camilla Charlton, Dissertation Advisor
  • Jason Thomas Wright, Committee Chair
  • Caryl Ann Gronwall, Committee Member
  • Suvrath Mahadevan, Committee Member
  • Christopher Howard House, Outside Member
  • Astronomy
  • Quasar Absorption Lines
  • Circumgalactic Medium
  • Galaxies
  • Spectroscopy
To understand the formation and evolution of galaxies, it is essential to understand the gas that exists in the vicinity of galaxies. Mg II absorption systems probe the circumgalactic medium (CGM) around galaxies and can provide hints to the nature of the gas. To fully understand the complex structure of the CGM, it is necessary to investigate not only low-ionization transitions such as those of Mg II, but also transition lines associated with more diffuse high-ionization material. We report our analysis of six intervening quasar absorption line systems, which probe the CGM at redshifts 0.4 < z < 1.0. We model these systems as multi-phase material and constrain the parameters of our model by fitting to many ionization transitions including those of C II, C III, C IV, Fe II, Mg I, Mg II, N II, N III, N V, O IV, O VI, S V, Si II, Si IV, and the H I Lyman series. We use the microphysics code Cloudy extensively to model the systems. We develop an efficient method for visually determining valid Cloudy models, which can be used for any other absorption systems in future studies. This method allows for careful examination of the absorption systems on a cloud-by-cloud basis in order to better understand the substructure inherent within the system. Our systems feature a system at redshift z = 0.48 with a multi-phase structure that includes gas with metallicity [Fe/H] < -2.5 as well as low- and high-ionization gas with metallicity near solar. Another system features a low-ionization subsystem with detected molecular hydrogen that also has significant O VI detected at nearly the same velocity. Another system can be modeled with only one phase of material that contributes to the absorption detected in both traditionally low-and high-ionization transitions. The physical properties of several other systems are discussed, with a focus on new constraints from the Hubble Space Telescope/Cosmic Origins Spectrograph's coverage of high-ionization transition lines. We argue that to fully understand the parameters of absorption systems such as metallicity or ionization state, it is important to carefully consider the substructure of the systems on a cloud-by-cloud basis. We discuss the dangers of "fast" analysis by averaging over all the components of a system. In general, without a careful cloud-by-cloud approach, it is possible that the extracted physical parameters of an absorption system do not match the actual properties of the real gas that caused the absorption.