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 [O ii] 3727 Åand [Ne v] 3426 Åemission 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 most particularly by a nearby cluster elliptical galaxy. Using the cluster mass model, we can compute its intrinsic shape. Requiring that the outer ring follows an ellipse we put constraints on the M/L ratio of the nearby galaxy and derive a magnification factor of 2.5 $\pm$ 0.2. The absolute luminosities of the source are then $L_B=1.310^{12}{L}_{B\odot }$ and $\nu$ L${}_{\nu }\simeq 4.10^{10}$ L${}_{\odot }$ in the mid-IR.

arXiv comment5 pages, 5 figures, uses aa.cls, accepted to A&A Letters. Minor changes, Figure 1 revisited and typos added