A gravitational lens in the Hubble Deep Field

Cosmologically distant galaxies ought to act as multiply imaging gravitational lenses for a fraction $\sim0.002-0.005$ of background sources.$^{\cite{turner,blandford,schneider}}$ This prediction is being borne out by surveys of flat spectrum radio sources$^{\cite{patnaik,myers}}$ and optical surveys.$^{\cite{maoz,glazebrook,ratnatunga}}$ The incidence and character of strong gravitational lenses provide an important constraint on the source redshift distribution and world model at magnitudes too faint for direct spectroscopy.$^{\cite{kneib,kochanek92}}$ Recent observations of the Hubble Deep Field (HDF)$^{\cite{williams}}$ permit the optical lensing rate to be estimated in a uniform manner, using a single observation. In this letter, we report upon the discovery of HDF~J123652+621227, a candidate gravitational lens in the HDF. This lens may be multiply imaging several optical sources at different redshifts. If follow-up spectroscopy of the lens and the brightest image confirms this hypothesis, observations of this system alone can be used to obtain an estimate of the redshift distribution at extremely faint flux levels.