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The Metallicity Distribution of the Circumgalactic Medium of Galaxies at z < 1

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posted on 2018-01-10, 00:00 authored by Christopher B. Wotta

This dissertation is composed of two related projects focused on assessing the metallicity distribution of circumgalactic gas at low redshift. The metallicity of the gas is a key property to characterize, since it is a direct diagnostic of the level of enrichment of the gas. This, in turn, informs us on the origin of the gas, e.g., metal-enriched outflows or inflows or metal-poor inflows. To determine the metallicity distribution of this gas, we assemble the largest samples to date of QSO absorbers known to probe circumgalactic medium (CGM) gas at z < 1; these absorbers are known as Lyman-limit systems (LLSs) and partial Lyman-limit systems (pLLSs). My dissertation work consists of two main surveys: (1) a blind survey of 55 CGM absorbers toward 140 QSOs (Chapters 2 and 3) and (2) a survey aimed to increase the sample of absorbers in the range 16.9 < log N(H I) < 19 (Chapters 4 and 5). The second survey was directly motivated by the results of the blind survey, which suggested a change in the metallicity distribution between the pLLSs and LLSs; however, the sample of LLSs in the blind survey was not large enough to robustly assess that conclusion.

As part of this work, we develop a new method to robustly derive the metallicity of ionized gas using only low-resolution spectra of H I and Mg II (Chapter 2); this significantly reduces the observational cost of such studies by a factor ~10. In the second part of my thesis, we also apply a Bayesian MCMC approach to determine the metallicity, which in particular provides more robustly-derived errors on the metallicities.

My dissertation shows unequivocally that metal-poor absorbers (gas with less than 10% or even 1–2% solar) are not rare in the z < 1 universe, contrary to what was previously thought. About 60% of the CGM absorbers (as probed by pLLSs and LLSs) are metal-poor. Galaxies are therefore host to large reservoirs of cool, dense, low-metallicity gas, even down to z < 1. We demonstrate a strong evolution of the metallicity with N(H I) between 16 and >21 dex, i.e., from very ionized to neutral gas. Across this range, the metallicity distribution changes from a unimodal distribution at log N(H I) > 19 (with an average around 25% solar metallicity) to a much broader and more complicated distribution at lower N(H I) (where we find a distribution that is at least bimodal with prominent peaks at 30% and 2–5% solar metallicity, and possibly another peak at <1% solar metallicity).

These findings provide new constraints for cosmological simulations that we discuss in Chapter 5. We argue that the low-metallicity CGM absorbers have properties consistent with cold accretion flows from the IGM seen in cosmological simulations. The high-metallicity CGM absorbers likely probe outflows from the host galaxy, tidally-stripped gas from satellite galaxies, or recycling flows.

History

Date Created

2018-01-10

Date Modified

2018-10-30

Defense Date

2017-11-03

Research Director(s)

Nicolas Lehner

Committee Members

Peter Garnavich Tan Ahn Grant Mathews

Degree

  • Doctor of Philosophy

Degree Level

  • Doctoral Dissertation

Program Name

  • Physics

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