We searched for star formation activity associated with high- z Damped Ly \alpha systems ( DLAs ) with Subaru telescope .
We used a set of narrow-band ( NB ) filters whose central wavelengths correspond to the redshifted Ly \alpha emission lines of targeted DLA absorbers at 3 < z < 4.5 .
We detected one apparent NB-excess object located 3.80 arcsec ( \sim 28 h _ { 70 } ^ { -1 } kpc ) away from the quasar SDSS J031036.84+005521.7 .
Follow-up spectroscopy revealed an asymmetric Ly \alpha emission at z _ { em } = 3.115 \pm 0.003 , which perfectly matches the sub-DLA trough at z _ { abs } = 3.1150 with log N ( H i ) /cm ^ { -2 } = 20.05 .
The Ly \alpha luminosity is estimated to be L _ { Ly \alpha } = 1.07 \times 10 ^ { 42 } erg s ^ { -1 } , which corresponds to a star formation rate of 0.97 M _ { \odot } yr ^ { -1 } .
Interestingly , the detected Ly \alpha emission is spatially extended with a sharp peak .
The large extent of the Ly \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 H i clouds in the circumgalactic medium .
This large Ly \alpha extension , which is similar to those found in Rauch et al .
( 50 ) , could be the result of complicated anisotropic radiative transfer through the surrounding neutral gas embedded in the DLA .