Unveiling the nature of the Cloverleaf lens-system: HST/NICMOS-2 observations

Abstract

We present new elements in the identification of the Cloverleaf lens-system based on recent NICMOS-2/F160W observations. After a careful PSF subtraction of the 4 images of the quasar, the residual H_F160W image reveals the presence of a faint object (H ~ 20.5) amid the 4 quasars. This object corresponds to a single galaxy that we identify as the primary lens of the lens-system. From the R, I and H photometry of the galaxies surrounding the Cloverleaf we also derive a photometric redshift. We find that most of them are consistent with belonging to a galaxy cluster/group with mean redshift /line{z}=0.9 +/- 0.1: they are likely to be the other component in the lens-system that provides the additional ``external" shear. Furthermore, we detect 2 very red objects (I-H ~ 4): the faintest one has no observed optical (R_F702W and I_F814W) counterpart, and the brightest has a predicted redshift z ~ 2, and may be identified with one of the Cloverleaf absorbers. This gravitational-lens system constitutes an excellent target for imaging/spectroscopy with the new generation of 8m ground-based telescopes. Based on observations obtained with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under the NASA contract NAS 5-26555.