A Galactic Wind at z=5.190

Abstract

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^''×20^''. 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 3.0×10^-17 ergs cm^-2 s^-1 corresponds to a luminosity of 9.0×10⁴² 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. Based on observations made at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.