자료유형  

 학위논문

Control Number  

0017162668

International Standard Book Number  

9798342718196

Dewey Decimal Classification Number  

523

Main Entry-Personal Name  

Hu, Teng.

Publication, Distribution, etc. (Imprint  

[S.l.] : University of California, Santa Barbara., 2024

Publication, Distribution, etc. (Imprint  

Ann Arbor : ProQuest Dissertations & Theses, 2024

Physical Description  

198 p.

General Note  

Source: Dissertations Abstracts International, Volume: 86-05, Section: B.

General Note  

Advisor: Hennawi, Joseph.

Dissertation Note  

Thesis (Ph.D.)--University of California, Santa Barbara, 2024.

Summary, Etc.  

요약One of the great successes of modern cosmology is the percent-level concordance between theory and observations of the intergalactic medium (IGM) at z ≳ 1.7. Yet, the Lyα forest at z 1.7, which can only be studied via HST UV spectra, has pointed out a puzzling discrepancy, i.e., the Doppler b-parameters of these absorption lines are, on average, ∼ 10 km/s wider than those in any existing hydrodynamic simulation. This discrepancy implies that the low-z IGM might be substantially hotter than expected, contradicting one of the fundamental predictions in current cosmology that the IGM should cool down owing to the Hubble expansion after He II reionization (z 2.5). Moreover, the IGM thermal state degenerates with its ionization state characterized by the UV background (UVB) photoionization rate, ΓHI, which dictates the abundance of the Lyα absorbers, dN/dz. Such a degeneracy requires any reliable measurement to adopt a careful statistical inference procedure. To overcome these difficulties, in this thesis, a novel machine-learning-based inference framework is employed to jointly measure the thermal and ionization state of the IGM, using the 2D distribution of b-parameter and H I column density and dN/dz. This method effectively resolves the degeneracies between the thermal and ionization state of the IGM and achieves high precision, even with limited-sized data. I apply this method to 94 archival HST COS and STIS quasar spectra distributed across the seven redshift bins, yielding a comprehensive evolutionary history of the IGM thermal and ionization state at z 1.5. The results suggest that the IGM may be significantly hotter than previously expected at low-z and is potentially isothermal, with IGM temperature at mean density, T0 ∼ 30, 000K and power-law index of the temperature-density, γ ∼ 1.0 at z = 0.1. The inferred thermal history suggests that this unexpected IGM temperature possibly emerges around z ∼ 1. Additionally, while the ΓHI measurements align with the theoretical model at z ∼ 1, the values measured at z 0.5 are substantially lower than predicted, posing challenges to low-z UV background synthesis models.

Subject Added Entry-Topical Term  

Astrophysics.

Subject Added Entry-Topical Term  

Astronomy.

Subject Added Entry-Topical Term  

Physics.

Subject Added Entry-Topical Term  

Computational physics.

Index Term-Uncontrolled  

Absorption lines

Index Term-Uncontrolled  

Cosmology

Index Term-Uncontrolled  

Intergalactic medium

Index Term-Uncontrolled  

Machine learning

Index Term-Uncontrolled  

UV background

Added Entry-Corporate Name  

University of California, Santa Barbara Physics

Host Item Entry  

Dissertations Abstracts International. 86-05B.

Electronic Location and Access  

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Control Number  

joongbu:654227