We report on the discovery of a very narrow-line star forming object beyond redshift of 5 .
Using the prime-focus camera , Suprime-Cam , on the 8.2 m Subaru telescope together with a narrow-passband filter centered at \lambda _ { c } = 8150 Å with passband of \Delta \lambda = 120 Å , we have obtained a very deep image of the field surrounding the quasar SDSSp J104433.04 - 012502.2 at a redshift of 5.74 .
Comparing this image with optical broad-band images , we have found an object with a very strong emission line .
Our follow-up optical spectroscopy has revealed that this source is at a redshift of z = 5.655 \pm 0.002 , forming stars at a rate \sim 13 ~ { } h _ { 0.7 } ^ { -2 } ~ { } M _ { \odot } yr ^ { -1 } .
Remarkably , the velocity dispersion of Ly \alpha -emitting gas is only 22 km s ^ { -1 } .
Since a blue half of the Ly \alpha emission could be absorbed by neutral hydrogen gas , perhaps in the system , a modest estimate of the velocity dispersion may be \gtrsim 44 km s ^ { -1 } .
Together with a linear size of 7.7 h _ { 0.7 } ^ { -1 } kpc , we estimate a lower limit of the dynamical mass of this object to be \sim 2 \times 10 ^ { 9 } M _ { \odot } .
It is thus suggested that LAE J1044 - 0123 is a star-forming dwarf galaxy ( i.e. , a subgalactic object or a building block ) beyond redshift 5 although we can not exclude a possibility that most Ly \alpha emission is absorbed by the red damping wing of neutral intergalactic matter .