  Using galaxies as background light sources to map intervening Lya absorption
is a novel approach to study the interplay among galaxies, the circum-galactic
medium (CGM), and the intergalactic medium (IGM). Introducing a new measure of
z = 3.1$ HI Lya absorption relative to the cosmic mean, Delta_NB497, estimated
from photometric data of star-forming galaxies at 3.3 < z < 3.5, we have made
two-dimensional Delta_NB497 maps in the z = 3.1 SSA22 proto-cluster region and
two control fields (SXDS and GOODS-N fields) with a spatial resolution of ~ 5
comoving Mpc. The Delta_NB497 measurements in the SSA22 field are
systematically larger than those in the control fields, and this HI absorption
enhancement extends more than 50 comoving Mpc. The field-averaged (i.e., ~50
comoving Mpc scale) Delta_NB497 and the overdensity of Lya emitters (LAEs) seem
to be correlated, while there is no clear dependency of the Delta_NB497 on the
local LAE overdensity in a few comoving Mpc scale. These results suggest that
diffuse HI gas spreads out in/around the SSA22 proto-cluster. We have also
found an enhancement of Delta_NB497 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 Lya emission, which suggests that the Lya escape fraction from
galaxies depends on hydrogen neutrality in the CGM.
