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

Abstract

We present a study of a very peculiar object found in the field of the cluster-lens Abell 370. This object displays, in HST imaging, a spectacular morphology comparable to nearby ring-galaxies. From spectroscopic observations at the CFHT, we measured a redshift of z=1.062 based on the identification of [Oii] 3727 Angstroms and [Nev] 3426 Angstroms emission lines. These emission lines are typical of starburst galaxies hosting a central active nucleus and are in good agreement with the assumption that this object is a ring 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 particular by a nearby cluster elliptical galaxy. Using our cluster mass model, we 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 magnification factor of 2.5 +/- 0.2. The absolute luminosities of the source are then L_B = 1.3 \ 10(12) L_{B sun} and nu L_nu =~ 4. 10(10) L_sun in the mid-IR. Based on observations with the NASA/ESA {\it Hubble Space Telescope\/} obtained from the data archive at the Space Telescope European Coordinating Facility, with {\it ISO}; an ESA project with instruments funded by ESA Member States with the participation of ISAS and NASA, and with the Canada-France-Hawaii Telescope at Mauna Kea, Hawaii, USA.