The Giant Proto-Galaxy cB58; an Artifact of Gravitational Lensing?

The proto-galaxy, cB58, was discovered in the CNOC survey of cluster redshifts. Absorption features reveal that this system is at a redshift of $z=2.72$, implying an absolute magnitude of ${\mathrm{M}}_{\mathrm{v}}\sim -26$, and a star-formation rate of $4700{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$, making it the most ``active’’ star-forming galaxy. This proto-galaxy is observed to lie close $(\sim 6\text{scnp})$ to a central cluster galaxy at $z=0.373$. The X-ray properties of the cluster suggest that its mass, and therefore its lensing potential, could be greater than that found using a virial analysis. In this Letter we argue that the phenomenal properties of this proto-galaxy are due to the gravitational lensing effect of the foreground cluster, and the unlensed properties of the source are typical of high-redshift star-forming systems.