PhD Defense by Ting-Yi Lu

PhD Thesis: Link for the PhD Thesis will be updated, when it is uploaded by the NBI-Education team - PhD theses - Astrophysics – Niels Bohr Institute - University of Copenhagen

Title: Mapping the reionization morphology with Lyman alpha emission: Lyman-Alpha visibility and ionized bubbles around reionization-era galaxies

Abstract: After recombination, the intergalactic medium (IGM) was fully neutral and highly opaque to photons with λ < 1216 Å. The first stars and galaxies formed within the neutral IGM. The energetic radiation from these galaxies ionized hydrogen in the IGM around them, creating ionized bubbles, marking the onset of the cosmic reionization era. As galaxies continued to form and emit radiation, more ionized bubbles emerged, expanded, and merged. Eventually, the IGM became fully ionized and transparent to λ < 1216 Å photons.

When and how galaxies formed and reionized the universe remain open questions in astronomy. The reionization morphology—the spatial and size distribution of ionized bubbles— traces the properties of early galaxies and can be used to address these key questions. Excitingly, the unprecedented depth and wavelength coverage of the James Webb Space Telescope (JWST) enable us to discover and observe the rest-frame UV to NIR spectra of galaxies up to the early stages of reionization (z∼14). The Lyα emission from reionization-era galaxies can be used to probe the ionization state of the IGM surrounding them. Several recent JWST observations have reported detections of Lyα emission from z ≥ 7 galaxies, detections that cannot be explained by a uniformly highly neutral IGM beyond the midpoint of reionization. These discoveries suggest that the galaxies reside in early ionized bubbles, prompting an evaluation of the relationship between observable galaxies and ionized structures and opening a new window into reionization morphology mapping. In this thesis, I present and analyze Gpc-scale semi-analytical simulations to pave the way for inferring ionized bubble sizes from Lyα observations and for mapping reionization morphology.

In the first project (Lu et al., 2024a), we conducted Gpc-scale reionization simulations to study the realistic distribution of ionized bubble sizes around JWST-detectable galaxies, providing a theoretical baseline for interpreting recent and upcoming observations of Lyα from reionization-era galaxies. We also investigated whether detections of Lyα emission from z > 7 galaxies can be explained by enhanced Lyα visibility due to galaxies residing in large ionized bubbles. We found that JWST-detectable galaxies (MUV ≲-17), which trace biased halos compared to the typical reionization-era galaxy population, reside in bubbles with larger characteristic sizes than the global bubble population in our simulations. Consequently, the Lyα visibility of observable galaxies should be higher due to their larger-than-average bubble environments. JWST detections of Lyα from z < 8 galaxies are expected, as our simulations suggest their surrounding bubbles are large enough at the observationally constrained ¯xHI to allow significant Lyα transmission. The z > 8 Lyα detections, however, can only be explained with a massive-halo-driven reionization model, a lower-than-expected ¯xHI, or intrinsically strong Lyα emission from the sources.

In the second part (Lu et al., 2024b), we developed the first Lyα-based methods that make use of ensembles of galaxies to map ionized bubbles both across the plane of the sky and along the line of sight using our reionization simulations. These methods will enable systematic constraints on reionization morphology with the increasing number of Lyα observations. Our methods detect bubble edges by leveraging the spatial variation in Lyα transmission among galaxies near the bubble edges. To accurately recover transmission profiles and identify bubble edges, we require a Lyα equivalent width detection limit of∼30Å, a systemic redshift precision of Δz ∼ 0.0015, and a galaxy number density of ∼0.004cMpc−3, all achievable with deep JWST NIRSpec observations.

With the improved understanding of the galaxy–ionized bubble connection across reionization models and the Lyalpha-based morphology mapping methods developed in this thesis, we are well positioned to interpret upcoming JWST observations and ultimately answer when and how reionization occurred.

Supervisor:

  • Charlotte Mason, Niels Bohr Institute, University of Copenhagen

Co-Supervisor:

  • Anne Hutter, Niels Bohr Institute, University of Copenhagen

PhD Evaluation Committee:

  • Matthew Hayes, Stockholm University
  • Benedetta Ciardi, MPA, Garching
  • Sune Toft, Niels Bohr Institute, University of Copenhagen

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*There will be a reception following in the "Julie Vinter Hansen" atrium at NBB (Tower I, ground floor)