Photometric IGM tomography with Subaru/HSC: the large-scale structure of Ly$\alpha$ emitters and IGM transmission in the COSMOS field at $z\sim 5$

We present a novel technique called “photometric IGM tomography” to map the intergalactic medium (IGM) at $z\simeq 4.9$ in the COSMOS field. It utilizes deep narrow-band (NB) imaging to photometrically detect faint Ly$\alpha$ forest transmission in background galaxies across the Subaru/Hyper-Suprime Cam (HSC)'s $1.8\mathrm{s}\mathrm{q}\mathrm{.}\mathrm{d}\mathrm{e}\mathrm{g}$ field of view and locate Ly$\alpha$ emitters (LAEs) in the same cosmic volume. Using ultra-deep HSC images and Bayesian spectral energy distribution fitting, we measure the Ly$\alpha$ forest transmission at $z\simeq 4.9$ along a large number ($140$) of background galaxies selected from the DEIMOS10k spectroscopic catalogue at $4.98<z<5.89$ and the SILVERRUSH LAEs at $z\simeq 5.7$. We photometrically measure the mean Ly$\alpha$ forest transmission and achieve a result consistent with previous measurements based on quasar spectra. We also measure the angular LAE-Ly$\alpha$ forest cross-correlation and Ly$\alpha$ forest auto-correlation functions and place an observational constraint on the large-scale fluctuations of the IGM around LAEs at $z\simeq 4.9$. Finally, we present the reconstructed 2D tomographic map of the IGM, co-spatial with the large-scale structure of LAEs, at a transverse resolution of $11h^{-1}\mathrm{c}\mathrm{M}\mathrm{p}\mathrm{c}$ across $140h^{-1}\mathrm{c}\mathrm{M}\mathrm{p}\mathrm{c}$ in the COSMOS field at $z\simeq 4.9$. We discuss the observational requirements and the potential applications of this new technique for understanding the sources of reionization, quasar radiative history, and galaxy-IGM correlations across $z\sim 3-6$. Our results represent the first proof-of-concept of photometric IGM tomography, offering a new route to examining early galaxy evolution in the context of the large-scale cosmic web from the epoch of reionization to cosmic noon.