A Faint Star-Forming System Viewed Through the Lensing Cluster Abell 2218: First Light at z~5.6?

Abstract

We discuss the physical nature of a remarkably faint pair of Lyman alpha-emitting images discovered close to the giant cD galaxy in the lensing cluster Abell 2218 (z=0.18) during a systematic survey for highly-magnified star-forming galaxies beyond z=5. A well-constrained mass model suggests the pair arises via a gravitationally-lensed source viewed at high magnification. Keck spectroscopy confirms the lensing hypothesis and implies the unlensed source is a very faint (I~30) compact (<150 pc) and isolated object at z=5.576 whose optical emission is substantially contained within the Lyman alpha emission line; no stellar continuum is detectable. The available data suggest the source is a promising candidate for an isolated ~10^6 solar mass system seen producing its first generation of stars close to the epoch of reionization.