The Sextet Arcs: A Strongly Lensed Lyman Break Galaxy in the ACS Spectroscopic Galaxy Survey toward Abell 1689

Abstract

We present results of the HST Advanced Camera for Surveys spectroscopic ground-based redshift survey in the field of A1689. We measure 98 redshifts, increasing the number of spectroscopically confirmed objects sixfold. We present two spectra from this catalog of the Sextet Arcs, images that arise from a strongly lensed Lyman break galaxy (LBG) at a redshift of z = 3.038. Gravitational lensing by the cluster magnifies its flux by a factor of ~16 and produces six separate images with a total r-band magnitude of r625 = 21.7. The two spectra, each of which represents emission from different regions of the LBG, show H I and interstellar metal absorption lines at the systemic redshift. Significant variations are seen in the Ly? profile across a single galaxy, ranging from strong absorption to a combination of emission plus absorption. A spectrum of a third image close to the brightest arc shows Ly? emission at the same redshift as the LBG, arising from either another spatially distinct region of the galaxy or from a companion galaxy close to the LBG. Taken as a group, the Ly? equivalent width in these three spectra decreases with increasing equivalent width of the strongest interstellar absorption lines. We discuss how these variations can be used to understand the physical conditions in the LBG. Intrinsically, this LBG is faint, ~0.1L*, and is forming stars at a modest rate, ~4 M? yr-1. We also detect absorption-line systems toward the Sextet Arcs at z = 2.873 and z = 2.534. The latter system is seen across two of our spectra.