Mapping the large-scale structure of the intergalactic medium (IGM) and its correlation with galaxies and active galactic nuclei (AGN) are critical for understanding how galaxies and AGN contribute to reionization. The key uncertainty lies in determining the fraction of Lyman continuum (LyC) photons escaping from galaxies into the IGM. To address this, we propose a novel photometric approach enabling 2D IGM tomography at z≃5.7 in the COSMOS. Utilizing extremely deep HSC narrow-band (NB) imaging, we determine a statistical measure of leaking LyC radiation from galaxies by cross-correlating Lyα emitters (LAEs) with nearby Lyα forest opacities along background sources. The proposed experiment also measures the auto-correlation of Lyα forest fluctuations, enabling us to constrain the contribution of AGN to reionization. Our reconstructed 2D tomographic map directly visualizes the spatial correlation between LAEs and the large-scale structure of the IGM, revealing the direct association between star-forming galaxies and ionized bubbles. This program provides critical Subaru NB data for the newly approved COSMOS-3D program, the largest (265 hr) JWST survey in Cycle 3, to establish a direct connection between galaxies/AGN and the IGM.
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