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

Abstract

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 ${\rm M_v \sim -26}$, and a star-formation rate of $4700 {\rm M_{\odot} yr^{-1}}$, making it the most ``active'' star-forming galaxy. This proto-galaxy is observed to lie close $\left(\sim 6\scnp\right)$ 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.