Massive Star Formation in a Gravitationally Lensed H II Galaxy at z = 3.357* ** ***

The Lynx arc, with a redshift of 3.357, was discovered during spectroscopic follow-up of the z ¼ 0:570 cluster RX J0848+4456 from the ROSAT Deep Cluster Survey. The arc is characterized by a very red RK color and strong, narrow emission lines. Analysis of HST WFPC2 imaging and Keck opticaland infrared spectroscopy shows that the arc is an H ii galaxy magnified by a factor of � 10 by a complex cluster environ- ment. The high intrinsic luminosity, the emission-line spectrum, the absorption components seen in Lyand C iv, and the rest-frame ultraviolet continuum are all consistent with a simple H ii region modelcontaining � 10 6 hot O stars. The best-fit parameters for this modelimply a very hot ionizing continuum ( TBB ' 80; 000 K), a high ionization parameter (log U '� 1), and a low nebular metallicity (Z=Z� ' 0:05). The narrowness of the emission lines requires a low mass-to-light ratio for the ionizing stars, suggestive of an extremely low metallicity stellar cluster. The apparent overabundance of silicon in the nebula could indicate enrichment by past pair-instability supernovae, requiring stars more massive than � 140 M� . Subject headings: cosmology: observations — galaxies: abundances — galaxies: high-redshift — gravitationallensing — H ii regions — stars: formation On-line material: color figures