The Discovery of a Very Narrow-Line Star Forming Obat a Redshift of 5.66ject

Abstract

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 $\hat{I}{»}_{\mathrm{c}}$ = 8150 à with passband of $\hat{I}”\hat{I}»$ = 120 à , 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 $\sim 13h_{0.7}^{-2}{M}_{\odot }$ yr${}^{-1}$. Remarkably, the velocity dispersion of Ly$\hat{I}\pm$-emitting gas is only 22 km s${}^{-1}$. Since a blue half of the Ly$\hat{I}\pm$ 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$-$0123 is a star-forming dwarf galaxy (i.e., a subgalactic object or a building block) beyond redshift 5 although we cannot exclude a possibility that most Ly$\hat{I}\pm$ emission is absorbed by the red damping wing of neutral intergalactic matter.

6 pages, 2 figures. ApJ Letters, in press