2002ApJ...570...92D

We report the serendipitous detection in high-resolution optical spectroscopy of a strong, asymmetric Lyα emission line at z=5.190. The detection was made in a 2.25 hr exposure with the Echelle Spectrograph and Imager on the Keck II telescope through a spectroscopic slit of dimensions 1''x20''. The progenitor of the emission line, J123649.2+621539 (hereafter ES1), lies in the Hubble Deep Field-North northwest flanking field, where it appears faint and compact, subtending just 0".3 (FWHM) with I_AB=25.4. The ES1 Lyα line flux of 3.0x10^–17 ergs.cm^–2.s^–1 corresponds to a luminosity of 9.0x10⁴² ergs.s^–1, and the line profile shows the sharp blue cutoff and broad red wing commonly observed in star-forming systems and expected for radiative transfer in an expanding envelope. We find that the Lyα profile is consistent with a galaxy-scale outflow with a velocity of v>300 km.s^–1. This value is consistent with wind speeds observed in powerful local starbursts (typically 10²-10³ km.s^–1) and compares favorably to simulations of the late-stage evolution of Lyα emission in star-forming systems. We discuss the implications of this high-redshift galactic wind for the early history of the evolution of galaxies and the intergalactic medium and for the origin of the UV background at z>3.