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Using galaxies as background light sources to map intervening Lya absorptionis a novel approach to study the interplay among galaxies, the circum-galacticmedium (CGM), and the intergalactic medium (IGM). Introducing a new measure ofz = 3.1$ HI Lya absorption relative to the cosmic mean, Delta_NB497, estimatedfrom photometric data of star-forming galaxies at 3.3 < z < 3.5, we have madetwo-dimensional Delta_NB497 maps in the z = 3.1 SSA22 proto-cluster region andtwo control fields (SXDS and GOODS-N fields) with a spatial resolution of ~ 5comoving Mpc. The Delta_NB497 measurements in the SSA22 field aresystematically larger than those in the control fields, and this HI absorptionenhancement extends more than 50 comoving Mpc. The field-averaged (i.e., ~50comoving Mpc scale) Delta_NB497 and the overdensity of Lya emitters (LAEs) seemto be correlated, while there is no clear dependency of the Delta_NB497 on thelocal LAE overdensity in a few comoving Mpc scale. These results suggest thatdiffuse HI gas spreads out in/around the SSA22 proto-cluster. We have alsofound an enhancement of Delta_NB497 at a projected distance < 100 physical kpcfrom the nearest z = 3.1 galaxies at least in the SSA22 field, which isprobably due to HI gas associated with the CGM of individual galaxies. The HIabsorption enhancement in the CGM-scale tends to be weaker around galaxies withstronger Lya emission, which suggests that the Lya escape fraction fromgalaxies depends on hydrogen neutrality in the CGM.

Imaging of diffuse HI absorption structure in the SSA22 proto-cluster region at z = 3.1