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The Absorption and Emission of Neutral Hydrogen around High-Redshift Star-Forming Galaxies


Chen, Yuguang (2022) The Absorption and Emission of Neutral Hydrogen around High-Redshift Star-Forming Galaxies. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/gsr9-9y83.


The circumgalactic medium (CGM) mediates the interaction between galaxies and their surrounding gas, and ultimately controls the evolutionary trajectory of its host galaxies. Among the CGM gas, neutral hydrogen (H I) constitutes an important component observationally. This thesis focuses on characterizing the circumgalactic H I around z = 2-3 star-forming galaxies using absorption and emission techniques in the Keck Baryonic Structure Survey (KBSS) fields.

Chapter 2 provides a new statistical measurement of the distribution and kinematics of H I in the CGM using Lyman-alpha (Lyα) absorption signatures. We constructed the KBSS Galaxy Pair Sample (KGPS), which contains > 200,000 distinctive foreground-background galaxy pairs with projected distances of 0.02-4 Mpc. We compared the observed H I absorption map with a cosmological zoom-in simulation, and found qualitative agreement. Furthermore, using an analytic model, we show that the observed H I distribution can be explained by a combination of outflow, accretion, and Hubble expansion.

Chapters 3 and 4 explore the Lyα emission, one of the strongest spectral features of H I, from the spatially resolved spectroscopy, by using the new high-sensitivity integral field unit, Keck Cosmic Web Imager (KCWI). We introduce the use of "cylindrically projected 2D (CP2D) spectra," which map the average Lyα spectral profile over a specified range of azimuthal angle, as a function of impact parameter, around galaxies. We find that the averaged CP2D spectrum is most consistent with the expected signatures of Lyα resonant scattering through outflowing gas.

Furthermore, Chapter 3 investigates the statistical relation between Lyα emission and its orientation relative to the galaxy major axis for 59 galaxies that have been observed with both KCWI and high-spatial-resolution imaging instruments. We find that the extended Lyα emission along the galaxy projected major and minor axes are statistically indistinguishable at projected distance ≾ 30 physical kpc, indicating that resonant scattering of Lyα by outflowing H I occurs in a statistically uniform halo, independent of galaxy orientation.

Chapter 4 provides ongoing research on the connection between Lyα emission in the CGM and the host galaxy properties. Our preliminary result from 110 star-forming galaxies at z ≃ 2-3 demonstrates that the host galaxy affects the production and escape of Lyα emission at ≾ 15 kpc as expected. However, the faint envelopes of Lyα emission reach to similar spatial distances (≳ 30 kpc) regardless of their host galaxies.

Finally, we document the standard data reduction procedure and data products of the KBSS-KCWI survey in Appendix A.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Galaxies; Interstellar Medium; Circumgalactic Medium
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Astrophysics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Steidel, Charles C.
Thesis Committee:
  • Kirby, Evan N. (chair)
  • Martin, D. Christopher
  • Hopkins, Philip F.
  • Mawet, Dimitri
  • Steidel, Charles C.
Defense Date:2 August 2021
Record Number:CaltechTHESIS:08232021-201252167
Persistent URL:
Related URLs:
URLURL TypeDescription adapted for Chapter 2. adapted for Chapter 3.
Chen, Yuguang0000-0003-4520-5395
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:14332
Deposited By: Yuguang Chen
Deposited On:30 Aug 2021 16:16
Last Modified:04 Aug 2022 22:04

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