Gravitational lensing in low-redshift clusters of galaxies: the arc-like object in Abell 3408 and its lensing interpretation

Abstract

We analyze the seldomly discussed lensing effects expected in low-z clusters (z = 0.05-0.15), using as an example the bright arc (z=0.073) discovered by Campusano and Hardy(1996) near the dominant cD galaxy of the cluster Abell 3408 (z=0.042). We present photometric and spectroscopic observations for both the dominant galaxy and the arc. The mass distribution in A3408 is modeled by scaled versions of the representative distributions derived from studies of clusters at higher redshifts. The two gravitational potentials considered are: i) a ``minimum'' mass case where the mass distribution follows the light profile of the central elliptical galaxy and, ii) a ``maximum'' mass case where a typical massive dark halo is added to the previous case. The observed arc is well reproduced by both models, but rather small magnifications of the source galaxy are implied. The source galaxy is tentatively identified in both the lensing and non-lensing scenarios as being a spiral. The smaller lensed spiral (14.6 h_50^{-1} kpc, M_B=-18.2) predicted by the dark halo model appears to fit the observations marginally better. Furthermore, we found that only the dark halo model predicts a measurable amount of weak shear in the images of faint background galaxies. We conclude that observations, under very good seeing conditions, of week shear in faint background galaxies in the direction of low-redshift galaxy clusters are possible. When the latter are combined with X-ray data, a powerful tool to probe the mass distribution in the very central region of galaxy clusters emerges.