Double-damped Lyα absorption:: A possible large neutral hydrogen gas filament near redshift z=1

We report the discovery of two damped Lyalpha systems (DLAs) near redshift z = 1 along a single quasar sight line (Q1727 + 5302) with neutral hydrogen column densities of N-H I = (1.45 +/- 0.15) x 10(21) and (2.60 +/- 0.20) x 10(21) atoms cm(-2). Their sight line velocity difference of 13,000 km s(-1) corresponds to a proper separation of 106 h(70)(-1) Mpc if interpreted as the Hubble flow (Omega(m) = 0.3, Omega(Lambda) = 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 approximate to 22 and approximate to32 h(70)(-1) 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-H, 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 Lyalpha absorption approximate to 37% of the time, but almost always N-H I > 10(19) atoms cm(-2). Consequently, this rare configuration of four high-N-H, absorbers with a total sight line velocity extent of 30,600 km s(-1) may represent a large filament-like structure stretching over a proper distance of 241 h(70)(-1) Mpc 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.