The Discovery of a Very Narrow Line Star-forming Object at a Redshift of 5.66

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Abstract
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 λc = 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 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 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.
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