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This paper investigates the origin of Lyman-α transmission spikes observed in quasar spectra during the late stages of cosmic reionization ($z > 5$). These rare, narrow regions of transmitted flux are often interpreted as signposts of nearby galaxies, under the assumption that local ionizing sources carve out transparent bubbles in the intergalactic medium (IGM). However, several observational studies have found little evidence for a positive correlation between spike positions and galaxies—raising the question of what sets their locations.

Lyman-α transmission spikes are narrow regions of high transmitted flux in quasar spectra observed at redshifts ($z > 5$), often interpreted as signposts of local ionizing sources. The prevailing hypothesis is that proximity to star-forming galaxies reduces the local neutral fraction, increasing transmissivity. However, However, several observational studies have found little evidence for a positive correlation between spike positions and galaxies, motivating a closer theoretical examination of the physics behind spike formation.

Using the Cosmic Reionization On Computers (CROC) simulations, we analyze the spatial and radiative environments of spike locations and decompose the ionizing radiation field into two components:

• $F_\nu$: the large-scale ionizing background arising from the integrated contribution of distant sources;
• $G_\nu$: the local ionizing field contributed by galaxies along the line of sight.

This decomposition enables a physically clean separation between large-scale and local radiative effects on the neutral hydrogen fraction. We find that the dominant factor controlling spike formation is gas density. The Ly$\alpha$ optical depth scales as $\tau_\alpha \propto n_{\mathrm{H}}^2 / \Gamma$, where $\Gamma$ is the ionization rate. In underdense regions, typical ionizing backgrounds can fully ionize the gas, enabling transmission. In contrast, overdense regions remain opaque regardless of the presence of local sources, as their higher densities sustain a higher neutral fraction despite elevated ionization rates.

These results explain why Lyman-$\alpha$ transmission spikes are not preferentially located near galaxies. Because galaxies reside in overdense environments with high recombination rates, their surroundings tend to remain optically thick. In contrast, spikes emerge from low-density regions where the background radiation field is sufficient to keep the gas ionized.

We conclude that the appearance and location of Ly$\alpha$ transmission spikes are primarily governed by gas density and modulated by the large-scale ionizing background $F_\nu$. The local radiation field $G_\nu$ plays a subdominant role. Rather than tracing the positions of nearby galaxies, transmission spikes encode information about the density structure and ionization topology of the IGM.