DISCOVERY OF STRONG LENSING BY AN ELLIPTICAL GALAXY AT z = 0.0345

摘要

We have discovered strong gravitational lensing by the galaxy ESO 325-G004, in images obtained with the Advanced Camera for Surveys on the Hubble Space Telescope. The lens galaxy is a boxy group-dominant elliptical at z = 0.0345, making this the closest known galaxy-scale lensing system. The lensed object is very blue (B - I_C ≈ 1.1), and forms two prominent arcs and a less extended third image. The Einstein radius is R_(Ein) = 1.9 kpc (～3″ on the sky), much smaller than the lens galaxy effective radius of 8.5 kpc. Assuming a high redshift for the source, the mass within R_(Ein) is 1.4 x 10~(11) solar mass, and the mass-to-light ratio is 1.8 (M/L)_(☉,I). The equivalent velocity dispersion is σ_(lens) = 310 km s~(-1), in excellent agreement with the measured stellar dispersion σ_v, = 320 km s~(-1). Modeling the lensing potential with a singular isothermal ellipse (SIE), we find close agreement with the light distribution. The best-fit SIE model reproduces the ellipticity of the lens galaxy to ～10%, and its position angle within 1°. The model predicts the broad features of the arc geometry as observed; the unlensed magnitude of the source is estimated at I_C ～ 23.75. We suggest that ～60 similarly massive elliptical galaxies within z < 0.1 will exhibit such a luminous, multiply imaged source.