ACS Observations of a Strongly Lensed Arc in a Field Elliptical

We report the discovery of a strongly lensed arc system around a field elliptical galaxy in Hubble Space Telescope ( HST ) Advanced Camera for Surveys (ACS) images of a parallel field observed during NICMOS observations of the HST Ultra-Deep Field. The ACS parallel data comprise deep imaging in the F435W, F606W, F775W, and F850LP bandpasses. The main arc is at a radius of 1 . ′′ 6 from the galaxy center and subtends about 120 ◦ . Spectroscopic follow-up at Magellan Observatory yields a redshift z = 0 . 6174 for the lensing galaxy, and we photometrically estimate z phot = 2 . 4 ± 0 . 3 for the arc. We also identify a likely counter-arc at a radius of 0 . ′′ 6, which shows structure similar to that seen in the main arc. We model this system and find a good fit to an elliptical isothermal potential of velocity dispersion σ ≈ 300 km s − 1 , the value expected from the fundamental plane, and some external shear. Several other galaxies in the field have colors similar to the lensing galaxy and likely make up a small group.