Coincident, 100 kpc scale damped Lyα absorption towards a binary QSO:: how large are galaxies at z ∼ 3?

We report coincident damped Ly alpha (DLA) and sub-DLA absorption at Z(abs) = 2.66 and Z(abs) = 2.94 towards the z similar to 3, 13.8 arcsec separation binary quasar SDSS 1116+4118 AB. At the redshifts of the absorbers, this angular separation corresponds to a proper transverse separation of similar to 110 h(70)(-1) kpc. A third absorber, a sub-DLA at Z(abs) = 2.47, is detected towards SDSS 1116+4118 B, but no corresponding high column density absorber is present towards SDSS 1116+4118 A. We use high-resolution galaxy simulations and a clustering analysis to interpret the coincident absorption and its implications for galaxy structure at z similar to 3. We conclude that the common absorption in the two lines of sight is unlikely to arise from a single galaxy, or a galaxy plus satellite system, and is more feasibly explained by a group of two or more galaxies with separations similar to 100 kpc. The impact of these findings on single line-of-sight observations is also discussed; we show that abundances of DLAs may be affected by up to a few tenths of a dex by line-of-sight DLA blending. From a Keck Echellette Spectrograph and Imager spectrum of the two quasars, we measure metal column densities for all five absorbers and determine abundances for the three absorbers with log N(HI) > 20. For the two highest N(H1) absorbers, we determine high levels of metal enrichment, corresponding to 1/3 and 1/5 Z (circle dot.) These metallicities are amongst the highest measured for DLAs at any redshift and are consistent with values measured in Lyman-break galaxies at 2 < z < 3. For the DLA at Z(abs) = 2.94 we also infer an approximately solar ratio of alpha-to-Fe peak elements from [S/Zn] = +0.05, and measure an upper limit for the molecular fraction in this particular line of sight of log f(H-2) < -5.5.