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Mapping the Diffuse Universe: Integral Field Spectroscopy of Galaxy Environments


O'Sullivan, Dónal Brendan (2020) Mapping the Diffuse Universe: Integral Field Spectroscopy of Galaxy Environments. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/6g0e-n897.


The population of galaxies we see today is the result of billions of years of gas inflows, outflows, mergers, and feedback. To develop any holistic picture of the origin and evolution of galaxies, we thus need to understand their environments. The circumgalactic and intergalactic media (CGM and IGM) - the gas around and between galaxies, respectively - represent a large part of this environment. However, this gas is extremely faint and thus difficult to observe, and only recently have we been able to image it directly. This thesis presents instrumental and observational work focused on revealing galaxy environments in the early universe.

Chapter 1 presents a brief history of our understanding of galaxies and an overview of our current picture of galaxy formation, including the role played by galaxy environments. In particular, it focuses on presenting the evolution of baryonic structures within a cosmological density field dominated by dark matter.

Chapter 2 presents instrumental work on the Keck Cosmic Web Imager (KCWI, Morrissey et al. 2018), a new integral field spectrograph (IFS) for the Keck-2 10m telescope designed to study faint, extended emission. As an introduction, I discuss the advantages and disadvantages of integral field spectroscopy for the application of studying galaxy environments, as well as an overview of the prototype instrument - the Palomar Cosmic Web Imager (PCWI, Matuszewski et al. 2010). This chapter focuses primarily on engineering work during the development and testing of KCWI, though I conclude with a brief comparison of PCWI and KCWI performance in measuring the CGM around a high-redshift QSO.

Chapter 3 presents the development of a software package designed to extract and analyze faint, extended emission in PCWI and KCWI data: CWITools. Although software is often an afterthought in astronomical and observational work, it is likely to become a primary barrier to conducting large IFS surveys of the CGM and IGM. This semi-automated analysis pipeline is presented and released publicly to empower future PCWI and KCWI studies.

Chapter 4 presents the FLASHES (Fluorescent Lyman-α Structures in High-z Environments) pilot survey, published as O'Sullivan et al. 2020. The FLASHES pilot survey is an IFS study of extended HI Lyman-α emission in the environments of 48 z = 2.3 - 3.0 QSOs. The FLASHES Survey is the core project of this thesis, enabled by the instrumentation in Chapter 2 and the analysis pipeline developed in Chapter 3. The pilot survey represents the first statistically significant (N ≳ 30) sample of direct CGM observations in its redshift range. As such, it provides the first direct constraints on the 2D morphology, surface brightness profiles, and spatially resolved kinematics of the CGM during this period.

Chapter 5 presents the first FLASHES follow-up study; deep IFS observations targeting extended Lyα 1216Å, NV 1240Å, CIV 1549Å, and HeII 1640Å emission from a subset of FLASHES pilot targets (O'Sullivan et al., in prep). Emission from metals in the CGM is expected to be an order of magnitude or more fainter than its Lyα, yet is a crucial ingredient in understanding the composition of the gas. Detecting this emission still requires multiple hours on 10m class telescopes. As such, large surveys of the multi-phase CGM remain extremely difficult to conduct. In this chapter, I present detections and upper limits of CGM metal emission around 8 FLASHES targets.

Chapter 6 presents engineering work on FIREBall-2 (the Faint Intergalactic Redshifted Emission Balloon, second generation), a high-altitude UV telescope and IFS targeting CGM emission in the low-redshift universe (z ≃ 0.7). FIREBall-2 is an ambitious project deploying a novel, electron-multiplying CCD designed to achieve ≳ 50% quantum efficiency in the UV. This technology represents an order of magnitude increase in sensitivity from the microchannel plates used in the GALEX space telescope. FIREBall-2 serves as both an observational project in its own right, studying the low-z CGM, and a pathfinder mission for future UV space missions.

Finally, Chapter 7 summarizes the contributions from this thesis and present a brief outlook on a few topics related to observations of galaxy environments.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Galaxy Evolution, Galaxy Formation, Integral Field Spectroscopy, Astrophysics
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Astronomy
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Martin, D. Christopher
Thesis Committee:
  • Steidel, Charles C. (chair)
  • Martin, D. Christopher
  • Hopkins, Philip F.
  • Hallinan, Gregg W.
  • Mawet, Dimitri
Defense Date:24 July 2020
Record Number:CaltechTHESIS:08042020-182952053
Persistent URL:
Related URLs:
URLURL TypeDescription adapted for ch. 4 instrument in work for ch. 6 instrument in work for ch. 2 study based on work in ch. 4 and 5
O'Sullivan, Dónal Brendan0000-0002-4959-9179
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:13846
Deposited By: Donal OSullivan
Deposited On:29 Sep 2020 23:14
Last Modified:06 Oct 2020 15:41

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