2003ApJ...596..797F

The Lynx arc, with a redshift of 3.357, was discovered during spectroscopic follow-up of the z=0.570 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 ∼10 by a complex cluster environment. The high intrinsic luminosity, the emission-line spectrum, the absorption components seen in Lyα and C IV, and the rest-frame ultraviolet continuum are all consistent with a simple H II region model containing ∼10⁶ hot O stars. The best-fit parameters for this model imply a very hot ionizing continuum (T_BB≃80,000 K), a high ionization parameter (logU≃-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_☉.