Using galaxies as background light sources to map intervening Ly α absorption is a novel approach to study the interplay among galaxies, the circumgalactic medium (CGM) and the intergalactic medium. Introducing a new measure of z = 3.1 HI Ly α absorption relative to the cosmic mean, ΔNB497, estimated from photometric data of star-forming galaxies at 3.3 ≲ z ≲ 3.5, we have made two-dimensional ΔNB497 maps in the z = 3.1 SSA22 protocluster region and two control fields (Subaru/XMM-Newton Deep Survey; Great Observatory Optical Deep Survey North) with a spatial resolution of ~5 comoving Mpc. The ΔNB497 measurements in the SSA22 field are systematically larger than those in the control fields, and this HI absorption enhancement extends more than 50 comovingMpc. The field-averaged (i.e. ~50 comovingMpc scale) ΔNB497 and the overdensity of Ly α emitters (LAEs) seem to be correlated, while there is no clear dependency of theΔNB497 on the local LAE overdensity in a few comoving Mpc scale. These results suggest that diffuse HI gas spreads out in/around the SSA22 protocluster. We have also found an enhancement of ΔNB497 at a projected distance < 100 physical kpc from the nearest z = 3.1 galaxies at least in the SSA22 field, which is probably due to HI gas associated with the CGM of individual galaxies. The HI absorption enhancement in the CGM-scale tends to be weaker around galaxies with stronger Ly α emission, which suggests that the Ly α escape fraction from galaxies depends on hydrogen neutrality in the CGM.
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