Massive star formation in a gravitationally lensed HII galaxy at z=3.357

The Lynx arc, with a redshift of 3.357, was discovered during spectroscopic follow-up of the z =0.70 cluster RX J0848+ 4456 from the ROSAT Deep Cluster Survey. The arc is characterized by a very red R - K color and strong, narrow emission lines. Analysis of HST WFPC2 imaging and Keck optical and infrared spectroscopy shows that the arc is an H II galaxy magnified by a factor of similar to 10 by a complex cluster environment. The high intrinsic luminosity, the emission-line spectrum, the absorption components seen in Lyalpha and C IV, and the rest-frame ultraviolet continuum are all consistent with a simple H II region model containing similar to 10(6) hot O stars. The best-fit parameters for this model imply a very hot ionizing continuum (T-BB similar or equal to 80, 000 K), a high ionization parameter (log U similar or equal to - 1), and a low nebular metallicity (Z/Z(.) similar or equal to 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 similar to140 M-..