The discovery of a very narrow line star-forming object at a redshift of 5.66

We report on the discovery of a very narrow line star-forming object beyond a 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 λ_c = 8150 Å with a passband of Δλ = 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 broadband 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 ± 0.002, forming stars at a rate ∼ 13 h _0.7^-2 M_⊙ yr^-1. Remarkably, the velocity dispersion of Lyα-emitting gas is only 22 km s^-1. Since the blue half of the Lyα emission could be absorbed by neutral hydrogen gas, perhaps in the system, a modest estimate of the velocity dispersion may be ≳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 ∼2 × 10⁹ M_⊙. 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 cannot exclude the possibility that most Lyα emission is absorbed by the red damping wing of neutral intergalactic matter.