A ring galaxy at z=1 lensed by the cluster Abell 370

We present a study of a very peculiar object found in the field of the clus ter-lens Abell 370. This object displays, in HST imaging, a spectacular mor phology comparable to nearby ring-galaxies. From spectroscopic observations at the CFHT, we measured a redshift of z = 1.062 based on the identificati on of [OII] 3727 Angstrom and [Nev] 3426 Angstrom emission lines. These emi ssion lines are typical of starburst galaxies hosting a central active nucl eus and are in good agreement with the assumption that this object is a rin g galaxy. This object is also detected with ISO in the LW2 and LW3 filters, and the mid Infra-Red (MIR) flux ratio favors a Seyfert 1 type. The shape of the ring is gravitationally distorted by the cluster-lens, and in partic ular by a nearby cluster elliptical galaxy. Using our cluster mass model, w e can compute its intrinsic shape. Requiring that the outer ring follows an ellipse we constrain the M/L ratio of the nearby galaxy and derive a magni fication factor of 2.5 +/- 0.2. The absolute luminosities of the source are then L-B = 1.3 10(12) L-B. and upsilon L-upsilon similar or equal to 4.10( 10) L. in the mid-IR.