  This paper reports the discovery of spatially-extended line-emitting nebula,
reaching to ~100 physical kpc (pkpc) from a damped Ly{\alpha} absorber (DLA) at
z_DLA=0.313 along the sightline toward QSO PKS1127-145 (z_QSO=1.188). This DLA
was known to be associated with a galaxy group of dynamical mass M_group ~3e12
M_sun, but its physical origin remained ambiguous. New wide-field integral
field observations revealed a giant nebula detected in [OII], H{\beta}, [OIII],
H{\alpha}, and [NII] emission, with the line-emitting gas following closely the
motions of group galaxies. One of the denser streams passes directly in front
of the QSO with kinematics consistent with the absorption profiles recorded in
the QSO echelle spectra. The emission morphology, kinematics, and line ratios
of the nebula suggest that shocks and turbulent mixing layers, produced as a
result of stripped gaseous streams moving at supersonic speed across the
ambient hot medium, contribute significantly to the ionization of the gas.
While the DLA may not be associated with any specific detected member of the
group, both the kinematic and dust properties are consistent with the DLA
originating in streams of gas stripped from sub-L* group members at <~25 pkpc
from the QSO sightline. This study demonstrates that gas stripping in low-mass
galaxy groups is effective in releasing metal-enriched gas from star-forming
regions, producing absorption systems in QSO spectra, and that combining
absorption and emission-line observations provides an exciting new opportunity
for studying gas and galaxy co-evolution.
