2004ApJ...609L..53T

We report the discovery of two damped Lyα systems (DLAs) near redshift z=1 along a single quasar sight line (Q1727+5302) with neutral hydrogen column densities of N_HI_=(1.45±0.15)x10²¹ and (2.60±0.20)x10²¹ atoms/cm2. Their sight line velocity difference of 13,000 km/s corresponds to a proper separation of 106 h^–1₇₀Mpc if interpreted as the Hubble flow (Ω_m=0.3, Ω_Λ=0.7). The random probability of such an occurrence is significantly less than 3%. Follow-up spectroscopy reveals neutral gas-phase Zn abundances of [Zn/H]=-0.58±0.15 (26.5% solar) and -1.32±0.28 (4.7% solar), respectively. The corresponding Cr abundances are [Cr/H]=-1.26±0.15 (5.5% solar) and -1.77±0.28 (1.7% solar), respectively, which is evidence for depletion onto grains. Follow-up IR images show the two most likely DLA galaxy candidates to have impact parameters of ~22 and ~32 h^–1₇₀ kpc if near z=1. They are significantly underluminous relative to the galaxy population at z=1. To investigate the possibility of additional high-N_HI_absorbers, we have searched the Sloan Digital Sky Survey database for z>1 quasars within 30' of the original sight line. Five were found, and two show strong Mg II-Fe II absorption near z=1, consistent with classical damped Lyα absorption ~37% of the time, but almost always N_HI_>10¹⁹ atoms/cm2. Consequently, this rare configuration of four high-N_HI_absorbers with a total sight line velocity extent of 30,600 km/s may represent a large filament-like structure stretching over a proper distance of 241 h^–1₇₀Mpc along our sight line, and a region in space capable of harboring excessive amounts of neutral gas. Future studies of this region of the sky are encouraged.