  We searched for star formation activity associated with high-z Damped
Lyman-alpha systems (DLAs) with Subaru telescope. We used a set of narrow-band
(NB) filters whose central wavelengths correspond to the redshifted Lyman-alpha
emission lines of targeted DLA absorbers at 3<z<4.5. We detected one apparent
NB-excess object located 3.80 arcsec (~28kpc) away from the quasar SDSS
J031036.84+005521.7. Follow-up spectroscopy revealed an asymmetric Lyman-alpha
emission at z_em=3.115+/-0.003, which perfectly matches the sub-DLA trough at
z_abs=3.1150 with logN(HI)/cm^-2=20.05. The Lyman-alpha luminosity is estimated
to be L(LyA)=1.07x10^42 erg s^-1, which corresponds to a star formation rate of
0.97 M_\odot yr^-1. Interestingly, the detected Lyman-alpha emission is
spatially extended with a sharp peak. The large extent of the Lyman-alpha
emission is remarkably one-sided toward the quasar line-of-sight, and is
redshifted. The observed spatially asymmetric surface brightness profile can be
qualitatively explained by a model of a DLA host galaxy, assuming a galactic
outflow and a clumpy distribution of HI clouds in the circumgalactic medium.
This large Lyman-alpha extension, which is similar to those found in Rauch et
al. (2008), could be the result of complicated anisotropic radiative transfer
through the surrounding neutral gas embedded in the DLA.
